The tgas are spliced up into icon references in various .brushes files in the Window folder. I don't remember off the top of my head which brushes file deals with that particular .tga, but it should have a fairly obvious name.
The brushes are in binary and not included in the new forge tools 3.0 download, so you'd have to convert them manually with converdata, just use brushes instead of entity for the command line parameter.
