A few people have been scammed on the site, Only use paypal to pay for items for sale by other members. If they will not use paypal, its likely a scam NEVER SEND E-TRANSFERS OF ANY KIND.
You need to host the photo somwhere else like photobucket. Once you've done that click the link icon and you'll copy the URL. All you need to do then is paste it directly to a reply in the forum window. Don't try and click the photo insert button and add the link there.