From: The Imperial Palace.
I hear good things about GIMP, and you can't argue with the price. I try to keep my picture width to no more than 1024, just in case people are browsing with older PCs. Try to save them as a jpeg. I try to keep my file size under 250K, partly because I post at another forum that insists on files of that size or lower; they load faster. Again, you can do this by "save as" a JPEG in your photo-editing software.
Like the Cube, I keep my game well ahead of my AAR. That helps me edit out the boring parts later. :) Also it helps me construct a more coherent narrative. E.g., I might fight three little skirmishes that end up being insignificant, and one big battle that's huge. Rather than report on all four, I might wait til I'm further on to assess which (if any) deserve forum space and time.
Personally, I use Photoshop, but I use it and other artist software professionally; it's not necessary for this. I save screenshots with both numpad-zero and with FRAPS; I'm not sure there's any quality difference between the two.
Cube: new avatar, eh? You don't look as gelatinous.