Viewer

• different sizing modes, or possibility to disable even the browser's default resizer.
  Beside the normal sizing modes there are two special:
  1. background mode (good for patterns)
  2. custom size, where you can manually set the dimensions
     Valid values (where x is a number with or without % mark):
     0 - means auto/original for both dimensions
     x[%] 0 - means auto for height and x for width (100% is the window's inner width)
     0 x[%] - means auto for width and x for height (100% is the window's inner height)
     Examples:
     20px width with reserved aspect ratio = "20",
     20 percent width, 50px height = 20% 50,
     reset to original size = 0
     This works in background mode too.
Key C cycles through the normal modes.
With left click you can jump between natural size and fit to window mode.
Holding down the left mouse button ("longpresssing") will switch between fit to width/height.
• custom background color, it's possible to set Opera skin elements too, so for instance you can have glass transparency with -o-skin('Window Skin') as background on Windows 7 (but not recommended for slower machines).
• custom image background color/image, accepts the CSS background property which means no limit.
• custom image border, this one uses CSS border property, but has a little restriction on border-width, maximum allowed is 100px
  
• panning, in Windows, by some reason Opera sometimes doesn't handle correctly the mouse movements, and while dragging the image it jumps randomly hither and thither. In preferences the "Panning threshold" solves this problem. If you have trouble with this, just set to a higher value. Smaller value means smoother panning, 0 means disabled.
  
• horizontal scrolling, just a little convenience. If image width is bigger than its height, then you don't have to point your cursor over horizontal scrollbar and drag or wheel, instead just use the mouse wheel on the page to scroll horizontally. For example, good for wide pictures, like panoramas. But if you think it is irritating, you can disable or fine tune on preference page. Holding down the X key will invert the scrolling mode.
• context menu, to easely access common features
• transformations, flipping (Q - vertical, W - horizontal) and rotation ([Shift + ] E - left, [Shift + ] R - right, 9 - Custom degree)
• zoom to selected area, holding Shift + left click + move will draw a rectangle, to move press Ctrl and move your mouse.
• zoom with mouse wheel, a convenient method to easily zoom to a selected point with your mouse wheel.

In preferences with Image informations you can customize the string you want to see in title of the document or on the status bar, it must start with "t:" (title), "s:" (status) or "b:" (both), and after the colon you can type anything what you want mixing with the following variables:
%ow - original width
%oh - original height
%ratio - current width / current height
%w - actual width
%h - actual height
%perc - percentage of resize
%url - full URL (this is the default, it's same as an empty string)
%name - name of the image
Example for title: "t:(%perc) %name"

For start, you can try out on these pictures:
http://upload.wikimedia.org/wikipedia/commons/9/9c/Ara_ararauna_(Linnaeus_1758).jpg
http://commondatastorage.googleapis.com/static.panoramio.com/photos/original/3157623.jpg
http://commondatastorage.googleapis.com/static.panoramio.com/photos/original/41461779.jpg
http://pennprolaw.com/wp-content/themes/pennprolaw/images/page-banner-2.jpg
http://www.loredanatritto.com/wp/wp-content/uploads/2010/08/Liquid-Sun_MD1.jpg

Hover zoom

Zoom linked or thumbnail images with moving the cursor over them.
If image is larger than the view-port, by default always will be fit to the window, but you can zoom to full size with pressing Z or Enter (problem-use).
Quite customizable and there is a Sieve that allows you to add rules for specific sites (which will work on images in viewer too, with these it can open images with higher resolution).

Tools

• easily open [background] images, favicon in new [background] tab
  On the preferences page under tools, you can read how to use or disable specific method.
• Image search
  New engines can be added in preferences under Tools / Image search (engine per line).
  Format: "[+]Name|search engine's URL with parameters". Now there is only one parameter: %url (full image address)
  Example: TinEye|http://www.tineye.com/search?url=%url
  If you put a "+" sign before the name (e.g., "+TinEye"), then it means that engine will be used when the shortcut (default key S) is pressed. Multiple entries can be enabled, then all of them will be used.
  With Ctrl + Alt + Right click on an image (on a web-page) will start the search.

After you installed, you can try out even on this page. Just refresh it, and for example do a right longpress on the page header or try on Mr. Norris . Yes, if you use longpress or Alt + right click then you will get a blank image. So, try with Ctrl + right click. And here are more playgrounds:
http://www.carto.net/papers/svg/samples/svg_html.shtml (embedded SVGs)
http://www.webdesignerwall.com/tutorials/how-to-css-large-background/ (this site has a lots of background images)
http://www.tizag.com/htmlT/htmlbackground.php (background attribute)
http://www.spartanicus.utvinternet.ie/embed.htm (embed, object)
http://en.wikipedia.org/wiki/Image_map (image maps)
http://testsuites.opera.com/object-fit/visual/001.html (misc)

Gallery

When an image is opened or "hoverzoomed" then pressing G will add it to the list without any questions. Shift + G will prompt for labels. Deleting images from the list: Ctrl + Right click on the thumbnail cell or Ctrl + Click to select some and the minus button from the button-list. Shift + Right click on a cell to edit the labels.
If possible, then two links will be saved with the thumbnails. The "Source", which is the address of the page where you've seen the image, and the "Target", which is the link address (if it was a link).
Search works on the address of the image and on the labels.

Grants

At Preferences / Grants you can tell which features allowed or blocked on specific sites.
It's possible to block everything, only a feature or just some sieve rules.

Basic syntax: "!" to block or "~" to allow: "*" for everything or URL or part of the URL
Example for blocking all sites (BTW, it's recommended to disable the features instead of doing this):

!:*

Example for blocking all sites except opera.com (actually it will allow any URL that contains the "opera.com" phrase, and yes, the order matters):

!:*
~:opera.com

To block a few sites:

!:http://www.google.com
!:opera.com
!:http://www.reddit.com/r/pics/

Advanced syntax: [vw|hz][!!|~~][#rule1, rule2, rule3]:[regular expression or same as above]
vw is for viewer, hz for hover zoom, if it's missing, then for both. When you double the grant operator (!, ~), then it means that you want use regular expression in the URL. For allowing/blocking specific sieve rules you can use the "#" mark, followed by the rule names separated by commas (,) and putting all this after the grant operator.
An example for blocking only hover zoom on all Google sites, except German and French:

hz!:http://www.google.com/
hz~~:^http://www\.google\.(?:de|fr)

Allowing only the "MyOpera" and "LinkedImages" rules here on my.opera.com:

~#MyOpera,LinkedImages:my.opera.com

For the extension's icon thanks goes to http://esk6a.deviantart.com/

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Known issues (I'm sure there is more, but these are a bit more common)
* rotated image has incorrect position both in viewer and in hz

image viewer:
* in some cases after moving the selection (freeZoom) on an image, the rectangle will lose its position

opening image:
* scanning images with Ctrl + right longpress is buggy

hover zoom:
* SVGs sometimes are not displayed at first time (maybe an Opera bug)
* zooming with "+ - 1, 2, 3" won't place the image to the right position, only after mouse cursor is moved

v0.8.10 - December 12
Good to know:
- Works only with Opera 11.60 or above.
- Fresh install recommended.
- The new Sieve contains total 162 rules, including some (corrected) filter from this set.

Full changelog can be found after install.