A wise computer animator once told me "It isn't the computer or the software that you have that makes the difference, it is what you do with what you have that counts."
Can't say I always believe him. I sure love faster computers.. But for now I'm going to have to live with this ghetto laptop.
I am a big fan of gimp, and I think it is a very powerful tool. But there are certainly other software programs out there that can do the job more efficiently.
If it is a matter of skill that concerns you can look at how bad I was/am >.> Looking at other peoples bad art is one of the ways I don't feel as bad about my own bad art! I am better now though than I was before.
There are some pretty amazing artists here. If you feel a lack of inspiration, browse through some of the things they have done.
If you need some inspiration/references you could always try to do a google search. Google has a way so that you can look for specific sized tiles. I've found good designs and ideas from looking up 12x12 goblins for example. Though you have to wade through a lot of images that are completely worthless. >.>
In any case, just hunker down and draw some stuff. If you feel you have come up short remember to be patient with yourself.
If you have trouble using certain aspects of gimp I know that there are other's here that use it as well. Or are at least familiar with it. Ask away, just try to be specific.