Not a big Gimp user so my knowledge is limited, but after you create a jpg of the symbol, you can use poser's material room to put it on the outfit. Normally you use the outfit's UV template. Here's a free tutorial (you have to join to download it but joining is free) by Mia 3D Design on ShareCG. There are also some tutorials here and at Deviant Art too.

http://www.sharecg.com/v/19993/related/3/Text-Entry-Tutorial/Creating-a-MAT-Pose-Pz2-file-for-Figure-Textures

This makes the file to point to the texture. A good text editor for this is either PhilC's PZ Editor, or the wonderful free Notepad++

The dust settled, thinking "what a fine home, at least for now" not realizing that doom would soon be coming in the form of a vacuum cleaner.