OK here is my advice (please don't take it personal or anything):
Your wallpaper seems to be a two layer composition or something. You should try to "blend" them together. For example, you could duplicate the layer that contains Eureka and then apply different effects that would blend it with the background.
Try using colours that compliment features of your character. For example, use colours that are similar to Eureka's hair. You've done a good job here anyway.
Try using more brushes and more colours. Personally I would make Eureka's face a bit darker because the background is a bit too dark. OR I'd use a lighter background. Anyway try to think of a theme for example, peace or love or something and then put it all together in your wallpaper. I actually can't do that. I try to blend the colours of my character with the background as much as possible but I rarely think of a theme. However, if you can the do it.
Anyway it's a good wallpaper and I hope my crap tips helped somehow.
Merci beaucoup for your comment.It 'is a good advice.This pic of a Eureka isone of my first colos(the 3°and I work with the Gimp^^ But i want to do my best and your comment can help me (sorry my english is very bad^^")You speak french?
I'm glad my comments helped you. Sorry I can only speak english, spanish and a bit of german but not french. I'm very fluent in english and spanish...i've studying german for 3 years but i'm not very good at it. Your english is not bad.