The idea is reasonable. A few comments:
* The login system is difficult to use. A user registration system on your site is really worth the effort; if not, at least supporting OpenID would be good. After logging in, returning to the page the user signed up on could also improve the flow.
* For a site like this, I'd expect that having a very refined visual design might matter to many users; the current visual design will probably be perceived as minimalistic. Minor details like curved borders and whitespace around important elements like drawings probably matter more to your audience than most.
* I'd suggest adding something similar to karma on HN or awards on StackOverflow. Make people compete for who can use the site the most and give them positive encouragement in the form of validation for doing things that benefit you.
In terms of getting users to the site, I think you need to decide on a business model; that will affect how much you can spend and be profitable. I think you probably have several groups of users:
1. Users who hear about the site, play with it, but aren't 'hooked' so don't return.
2. Users who get a link to an image on the site, look at it, and don't come back.
3. Regular contributors who come back to draw pictures.
4. Regular critics who look through the site looking for interesting stuff.
Everyone visiting your site potentially is of financial value to your business. You want to maximise their future present value (FPV - that is, how much a user is worth to you, taking into account that money in the future is worth less than money now because you aren't earning interest). A customer who regularly buys from you, or clicks on ads, is worth the most.
All of this assumes you have ways of extracting value from users. Suggestions:
* 'Get this drawing on a T-shirt / mug / print' (make sure you get users to agree to terms giving you a license to print their tiles).
* Advertising - not always a high income source, but still a viable one sometimes.
* Maybe some kind of freemium model - I'm not sure how it would work though.
Once you have an idea of how much a customer is worth, you can spend money driving them to your site - by advertising and PR - just make sure you spend less than what a customer is worth.
It is also could be worthwhile to add features to make it easier to share links to artwork on Twitter, Facebook, and so on - this is a good way to get virally referred users at no cost.