/vr/ - Retro Games

How do I edit a texture for use in Trenchbroom, or I suppose in general? I'd like to take pic related from the START wad and add some little touches to make it feel more aged in paint or something, but when I take it out it's .lmp, and changing that to any other format will make paint unable to open it (and it can't open its lmp incarnation of course).