http://cyangmou.deviantart.com/art/5-Tips-for-the-Aspiring-Pixel-Artist-457660938

I'll throw the old version of Aseprite into the mix. The latest versions are paid software, but 0.9.5 is extremely usable and unlike the free version of Graphics Gale, it has GIF support.

Edit: Forgot my standard warning. Aseprite scales pixels to 200% instead of 100% by default. To fix this, you have to change a single value in the .ini file found in the program's folder.
Close the program (if it's open) and change
ScreenScale = 2
to
ScreenScale = 1
Then relaunch the program.

http://www.humanbalance.net/gale/us/

I personally use GraphicsGale, and the free version is more than enough to make whatever you need in pixel art.
It should be much more easy to use than Gimp, too.

Good luck!

The free version of GraphicsGale is one of the most efficient you can get for free. The shareware is 17$ but I would say it is definitely worth the investment if you plan on making the game fully in the long run. Other than that, you can use Grafx2 which is also nice and simple or PS CS2 which is probably just as cluttered but very powerful with tons of tools like batch conversions.