How about using alt- and title-tags for those images?

right, meta tags are also missing at the moment. but generally speaking...its a javascript app, with dynamic content (filters) and only images... the googlebot is not going to apply filters. or maybe he is, i don't know.

The google bot won't touch any javascript, it will just follow links. You can consider a link to a directory to the different themes.

Googling "googlebot execute javascript" indicates otherwise.

I stand corrected. they use Javascript in a headless browser to generate the site previews. Detailed information on how Javascript is interpreted to index sites is hard to find. I would not count on Javascript to instruct Google Bot what to index.