Finally, you make sense!
Anyhow, here's how the cells work (each cell is 64*64 pixels):
01 02 03 04| 01: Head 02 - 06: Feet
05 06 07 08| 07: Body (no weps equipped) 08: Empty
09 10 11 12| 09 - 12: Optional weapon fire animation
13 14 15 16| 13 - 16: Optional weapon fire animation
Note that this is for player races ONLY. Other creatures use different type of gfx (which the player races used in Notrium 1.2).
Now, you can create the textures with any gfx editor (even paint), although if you use one that doesn't support transparency (e.g. paint), you'll need another editor to add that in.
You can use Flash MX 2004 for making the gfx (I do that. I also find it easier because of the amount of control over the things that make the gfx file... ).
And as you know (well, you're supposed to), the preferred texture format is .png (although I'm not sure which other formats are supported, could someone post a list?).