I'm going to be real critical here (I'm a bit of a graphic guy too, 4 years of photoshop experience)...
- There's no focal point. In a good picture, you need the eye to go somewhere. This doesn't have that.
- Colours are all over the place, there's not really a colour scheme, more randomly chosen colours everywhere.
- It's not particularly smooth, harsh colours and choppy images - it'd be better if they flowed.
- Lighting is a bit off - it's coming from the top where the joker is, and if you do want an effective focal point, that certainly isn't it.
Just a bit of constructive criticism, didn't mean to sound harsh or anything.
-Sam H