It looks like you may be limited to one image for free items so I'd like to see as many of the poses from a set without making them too small. They probably don't allow animated gifs, but if they do, that would be an alternative. Obviously include which figure(s) they're for and try to have a descriptive title - e.g. 'Shopping Poses' as opposed to 'Spending Spree.' :-)

A lot of vendors probably aren't into the ins and outs of marketing. It might pay the site to do some tutorials on keywords, making good promo renders etc. or even provide some kind of marketing assistance service. Someone could do that independently though whether they could make any money at it is open to question.   

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken