you don't really need anything more advanced than mspaint, anything else exists to streamline the process. there's not really any point in getting bogged down in advanced concepts like layers, palette indexing, hotkey remapping, index painting/color quantizing etc. until you're comfortable with the basics
with that said gimp is not a great tool for actually creating pixel art because it's designed primarily for non-pixel art work so you have a bunch of features that are essentially useless unless you are using a technique like index painting/resizing to make your pixel art
also there are two common pitfalls newcomers at pixel art tend to fall in:
-no hue ramps: this is pretty much what any beginner with no art background does (because if you have an art background you have some concept of a hue ramp). basically it's when an artist creates a ramp of shades by brightening/darkening a colour; this looks bad because it lacks depth and looks lifeless and dull. an example of no hue shifting vs hue shifting:
sourcethe top ramp is just one colour with various lightning values, the bottom one is a ramp where the colour moves towards a different colour (the green becomes slightly more blue as it becomes darker and more yellow as it becomes lighter.) hue shifting tends to add a lot of "pop". if you look at old tiles, the really old tiles tend to have no hue shifting when the newer ones (especially made by me or ontoclasm) have hue shifts everywhere.
-pillow shading: another pitfall one that people without art backgrounds tend to fall in: this is just improper understanding of shading. the easiest way to explain this is with a picture:
source (same as above)the left skull has no actual shading, the colour just gets brighter the further you are from a boundary. the right skull has actual shading. basically you need to learn proper shading
if you want good resources you can check:
http://pixeljoint.com/forum/forum_posts.asp?TID=11299http://pixeljoint.com/forum/forum_posts.asp?TID=5692http://www.vg-resource.com/thread-25474.htmlhttp://makegames.tumblr.com/post/426486 ... t-tutorialthey're not perfect, some of the stuff is pixel art culture that not all pixel artists agree on but it's a good starting point. learning a bit about colour theory (how to pick colours, the concept of hot/cold colours, complementary colours and why they matter) is also good
afterwards you should rummage through the pixelation forum and checki their WIPs section since it's populated with progress snapshots of work and there tends to be a lot of art breakdowns by the more experienced users