Very sorry, but I'd give you a 2.
The shape of their faces are irregular and therefore don't look right. It seems a bit simple as well. But I (sorta, I was slightly better) began like that when I started with digital art. Do you use a roller (or 'mechanical' mouse)? 'Cause they are the 2nd worst mice (tracker ball being the worst) you can ever use for digital art. Use a laser mouse and practice frequently.
Ways to improve:
1) Shapes! Anatomy! All that stuff. These things are high priority (in my opinio) when it comes to art.
2) Draw frequently! This is how you improve. Get a notepad and doodle at random times when you're at school or something.
3) Experiment! This is how you discover new ways of doing things and therefore improve.
4) Layers! I see you have not used any in those picture. GIMP does have a la
yer feature, so use it!
5) Only if you get SUPER serious (like Zebez said), buy a tablet. Get a HanWang tablet (I have one) instead of a Wacom one, it's cheaper, but only available in China. However, I only got mine after THREE YEARS of experience. So don't buy one unless you think you're AWESOME at it.