The most obvious solution here would be to vector the scan you used. That would fix the edges and clean up the grainy texture that I'm seeing right now.

Scan is very bad, full of noises. I suggested that you I cleaned it.
Good luck ;D.

This looks like a spoiled work for me... It looks like you just took a bad quality scan and put it on a background already existing and add your sign and the text on the left on it...

Anyway even if you did the background:
the character is grainy, full of noise and the colors are a bit twisted..
The background is full of pixels and jpeg artifacts from the saving. Make sure you use the "save for web" option when you save your wall.
The colors aren't really matching all together... For me the hair of the character isn't going that well with the pink of the background...

The text on the left is really not useful at all...

Your sign is a bit too high and the font is the dullest you can use : comic -___- It's borring.

Good luck