Right next to the choose URL box all the way on the right hand side there is a little square. Click that and you can then upload images from your computers. You could also just create a photobucket account, upload to there and use the URL that you get.
How do I post pictures in forums?

You should find all the answers you need in this thread:
http://www.chess.com/forum/view/help-support/inserting-a-picture-or-image
I see the icon box that looks like a tree. I know that is (or seems to be) the way to post a picture. I click on it and it asks for a URL. I just need to know a quick way to post pictures in these dialogue boxes...thanks for your help. I apologize for my lack of computer understanding! :-)