How do you add photos to the ad?

This is a question and answer from this board that is only a few threads down from your question.
Hope it helps.

Do a search on this topic. Change the number of days to 300 and see what has been posted here for help if the one I included doesn't tell you everything you need.

You can't upload a pic here.  You can link to the url of the picture(s) on your website.  

That is what the "picture URL" box below the "message" box is for.