This is my first time, so please give me your opnion about it !

http://xs.to/xs.php?f=tskiwall-00.jpg&h=xs108&d=06444

any tips or if you think that its ok to submit just say it ^^

I don't really see much wrong with it, I like the layout.
a few points you might want to look at is
-what size is it? it looks a little small, but min. is 1024x768 sorry if that is the size, just looked smaller to me
-your clouds are a bit realistic, if you've used cloud brushes, maybe try a different one,
-I've never seen a rainbow moon.
- the way her hair is in the moon, it's hard to tell if it's just sittin on a reflection, or if it's poor extraction, (just at the tips) look at it again and make sure, honestly, I think it just looks that way, cuz the extraction looks pretty good everywhere else.

MMMMmmmmMMm.. Its nice. One this I see wrong is that the left part of the skirt blends too much with the cloud and looks wierd to me.

Any better ? XD

The image could definitely be cleaned up...especially on the left side, with her hair, and big areas like her skirt. You can see all the random pixels. Otherwise, it looks pretty nice. THe lighting's kinda wierd, considering how the source of light's coming from BEHIND her...but ehh. It's not THAT big of a deal. XD

The big bat on the right side looks like a tree to me.