Do you want to upload a picture that is already online or a picture that is in your PC?
If it is in the net, you right-click on it, choose "picture properties" (or sth like that, my PC is in Greek) and copy the URL of the picture. URL = the adress that shows where the picture is (not the adress of the whole page that has the picture, it's a special adress).
Then come in this site, go where you write stuff, choose the little painting with the mountains that is above at the menu, and paste the URL there.
If you want to upload a pic that is in your PC, you have to get it in the net first. There are many sites where you can do this fast and easy.
For example, this one:
ImageShack® - Online Photo and Video Hosting
Then you get the URL that is ready in a box somewhere under the pic. Or you can post the link to the whole page, if you want.
Hope that helps