I say yes it should be required, however, I would take it further saying it should be something that can't be deleted from the ad until successful completion of the sale (that last part may not be possible, just a thought).
Only reason I think this is when there is a problem concerning condition or what was show in the ads pics, I thought I heard of some issues where a bad seller then either deletes the pic or changes it after the fact. If it something linked from say photobucket, I am not sure how you then catch that when a problem arises.
In general I save an offline copy of ads I purchase, but, if that was possible could be useful feature.
Now, outside of that, if we require price I don't see why requiring a pic is a bad thing either as using the argument about skipping over ads with out pics, same can be sad for those without prices too.