GIMP tips
 

Since the URLs provided have been for Photoshop only, here is some info for GIMP users:

GIMP has special filters useful for mapping under Filters > Map. For example, Filters > Map > Tile makes images tile seamlessly by applying a ghosting effect. Some Filters which generate stuff, like Filters > Render > Clouds > Solid Noise or those under Filters > Render > Pattern do have a "Tilable" option, which will take care that the result tiles seamlessly automatically.

To make your image tilable by hand, you can follow the same steps as in the guides above, look at the Tileable Textures tutorial or the more sophisticated Making Tileable & Seamless Textures tutorial for reference.

Re: GIMP tips
 

Thanks for the advice! http://forum.shrapnelgames.com/images/smilies/happy.gif

Re: GIMP tips
 

In case you want detailed guides, search for "tiling" or "tilable" textures. They're basically wraparound images that can be used e.g. as the wall or floor of a 3d building, and there are several detailed guides for doing those out of various pictures.

Re: GIMP tips
 

Well I just finished one myself.

Things that worked for me:
1. Patterns since they naturally fit nicely end to end AND look nicer than just a plain color.
2. Layers for everything.. makes changes easier
I had one layer for background which was a patern in GIMP then for land masses another with above and below that one other layers to smooth the sides (hey I bet you can do this easier but I'm no GIMP hero)
Then for other color landmasses (ice) anohter 3 layers
Then one for borders
Then one for white dots
3. Since the patterns took care of most wrap aourn problems that only left me the borders and the land one the wrap sides to do and then fill it in. I just used the positions you can see in GIMP to make sure the first bit of line was in the right place everytime and then after that was done I did the rest easily