Find something you like on the web.
Right click the picture. (should be a gif or jpg)
Click Properties
Highlight the URL
CTRL C to copy
Back to the forum and start a reply
Click the button
Position your cursor in the box that appears and CTRL V to paste the url.
Click OK
The pic url in your reply should now look something like:
[IMG]http://www.pics.com/picture.gif[/IMG]
If you want to add text, by all means do so
and then click Post Now
Not all gifs and jpgs work, but most do.