The design of each graphical image can and should be somewhat independent of the basic game design; this allows for reuse of pictures.
The first thing you should do is to check the image library on your machine. The image you're looking for may already be there, but perhaps under a different name. Even if you don't find it, you may notice an image that is close enough to be a good starting point. The Xconq image library presently includes hundreds of images, so the chances are pretty good that you'll find something useful.
Designing good images and patterns is a specialized and demanding category of artwork that I'm not going to go into here. My best advice is to learn from the pros, and don't be afraid to experiment.