Thanks everyone for your replies.

That looks like it produces some of the effects I'm after but unfortunately doesn't tell me how to achieve them. I really need to know how to produce the same effects using standard image filters.

This looks promising. I'm not sure how well it would work though as the curl/shadow/rotation effect needs to be applied against a background. I would need to provide a default background image to apply the effects against.

Now this looks exactly what I am after! Again, it's a shame they don't describe exactly what each slider does but by playing with it I may be able to replicate those effects. I think my next step is to try to replicate some of those effects in ToyCamera. If I get anywhere I'll post some of the results for comments.

I do plan on entering the competition with this app, I've already added it to the Wiki entry page. I hope I can get it complete on time...