How to promote an unusual/niche website?

Hi

I recently launched a new website (www.poptile.com), but it's a bit of an unusual idea - I won't go into too much detail (in case it's seen as blatant marketing!), but briefly: the idea behind the site is that visitors upload photos or images as "tiles" into themed "mosaic" designs, which will gradually grow into a unique work of art as more and more visitors upload. Initially the uploads will be free but I plan to make a small charge (a couple of dollars) once the site is established.

While someone will search for "online bookshop" if looking for an online bookshop, how on earth is my niche site likely to be found on a search engine?

I've followed various advice such as submitting to a number of directories, including some "niche" ones. I have also obviously submitted to the main search engines (both Yahoo and Google have now indexed it but not yet crawled). I have also started an adwords campaign for approx $50/month to try and jump-start the visitors. In 4 days it's given me around 100 visitors, mainly from content adwords rather than search adwords, but I'm finding that most of my keywords are "inactive for content" because they are too vague. But here-in lies the problem - it's such a niche site that I don't know how best to market and publicise it!

I'm keen on doing some press releases, however I don't really want to publicise the site too much until these mosaic designs are a little more "populated"! But it's a catch-22 situation - I need to get visitors in to upload, but can't do this unless I publicise. But if I publicise early then I run the risk of people thinking it isn't popular, as there are no uploads in the mosaics designs (unless I start putting up lots of my own uploads - did I say that?).

Any other suggestions guys?

Thanks in advance
Andy