The Falcon's Nest

To make a screenshot, you should press the PrintScreen key on your keyboard. This captures an image of your entire screen onto the clipboard. Alternatively, press Alt+PrintScreen to capture a snapshot of only the active application or dialog window. Then you can open your favorite image editor and paste the image in it. I use IrfanView, which is a free and excellent image-processing tool; but even Microsoft Paint would suffice.

In the pasted image, you can use the image tool's cut / crop / select / copy-paste utilities to remove unwanted parts of the image. Then save it in a nice compressed format such as png or jpg. Make sure that the resulting image is less than 100kB in size, that is the upper limit for uploading them onto the forums here. You might need to resize it (scale it to be smaller) and/or change its color depth to make it fit 100kB.

Once you have the image ready, use the 'Post reply' button on the thread page in the forum to add a post in an existing thread. Find the 'Attach file' section near the bottom of the text box. There is an entry-field to upload attachments -- specify the path to the image there. Once you upload the file, a preview image along will show up along with an Attach button. Click on the button to add the image to your post as an attachment.

Note that if you preview your post, you might need to specify the image attachment again. Once you submit, your post with the image will be available in the thread.