Cool game man. What you said in the description of the game was true! We got everything we need for having fun. Unfortunately, I was stuck at level 17. I couldn't kill all the enemies. If you plan to update the game with new features, I will suggest to add the possibility to have the solution or maybe just a clue on how to complete a level.
For the graphics issue, I didn't have any problem but, if you want to change them, I will suggest these (you probably know them) "Play With Your Peas". Players are more attracted by something "shining and beautiful".
Here is the link:
Play With Your Peas
And a screenshot:
<img src="http://dl.dropbox.com/u/22785599/OthersProjects/PlayYourPeas/PlayWithYourPeas.jpg" border="0">
They are free to use and you just need to put the name of the creator in your credits. In addition, you will only need a couple of them (just the peas). You could change the color of Peas to create enemies by using "The Gimp" (it's very easy to do that. You can also find tutorials about that on the internet).
Hope this helps! By the way, feel free to try also my game and leave me some comments. I will appreciate. You can play it through my signature.