The pictures can be attached, but need to be hosted externally, as far as I know.
And this is good, because otherwise chances are that the traffic for the forum will explode.
Maybe we can talk with Anthony about some sort of Template with fields for the different informations that should be given, and fields for pictures (where you just have to link the URL of the pic). This way all the offers would have the same structure, and all information would be easy to find for potential buyers.
The posting of images is in the sellers hands. As it is their sales pitch they are in charge of the content that is provided within.
With regard to templates (Jojo) I'm currently working on a few that sellers may use when selling. These will be available to sellers within the next week or so I hope. For your information the templates will include forum source code to produce nicely designed sales templates.