They're essentially direct draw surface images with a different header. Depending on what type of image you're going to edit, they'll also need to be swizzled. Decks must be swizzled.
There are tools for this. If you can create a targa image, index its color pallette, make it 32 bit again, save without RLE.
You should be able to use that image in defeat0rs tool.
Unfortunately defeat0rs tools are windows applications.