well, I never used gimp (with never I mean maybe one time) but here a quick tutorial:
1) data -> new file -> size 64x64 pixel 24bit color
2) fill picture with blue (the fill tool looks like a bucket I guess)
3) select airbrush and draw a slightly white blurry cirlce in the center
4) go into channels (there are some windows, like tools, layers and so on, and one should be called channels (RedGreenBlue/RGB))
5) create a new channel (if you don´t understand how, read the damn manual what it says about creating CHANNELs)
6) draw with the airbrush in the new CHANNEL (usually called alpha1) a white blurred circle on the black background
7) go back into the rgb channel
8) go on save file as
9) file name: plasma.tga, file type: targa bitmap (TGA) 32bit(!) not 24bit(or was it byte?)