This is how I just did it in Firefox:
This assumes you have a mouse with at least a left button and a right button...
1. Enter search term into Google Images
2. When results appear, what you first see is a page of thumbnails that you cannot link to. Single (left) click on the image you want and it will open up in a new, superimposed window. Now you can right click on the actual image and choose "copy image location".
3. You can now left click the grey X at the top right of the image window and you are back in the search results.
4. In A2K or other forum, in a posting/reply dialog, left click on the Img button, a pair of img tags will appear, left click to place the typing cursor (vertical line) click in between them and right-click and choose "Paste".
5. Click Reply