A smart theme shop might do something like have separate sections of themes entirely, behind a question on the front page.

Example, Dell has two separate stores, one for "home", one for "work". They both sell pretty much the same laptops and such, but with different marketing approaches and packages.

A theme shop could ask the user for the purpose of the theme right at the outset... for business, for a portfolio, for artists, for a blog, for a community group, for a church, etc. Then tailor the images and presentation on the store for each vertical. Same themes, different photos and marketing approaches. Might be a better way to think about it.

As long as you're not too hamfisted about it, it would definitely work better. A church looking for a theme doesn't need to see pictures of nuns and crosses and such, but they would respond better to more family-oriented stuff: picnics, people in parks, maybe a dog or two, that sort of general christian-ethos thing would be better images for such a page. Similarly, artists and portfolio pages are going to respond more to splashes of color, more chaotic imagery, much like paint girl above. Business and sales oriented approaches work better with blues and greys, statues, buildings and architecture, etc. That sort of thing.

The general idea here is that you want somebody looking at a theme demo to be seeing what they think their own site should generally look like, and that is going to change primarily depending on the purpose of their site, in their mind. So asking that at the outset is a good idea.