How do people 'photoshop' (or whatever they do with It) doomguy's arms on his weapons?

You rip them from existing in game weapons, use some sprite-drawing skill to fill in the gaps etc. People have made some templates:

Yeah,but...are these templates free-to-use like freedoom sprites or are them locked into a non-GNU license?

Well they ARE from Doom itself, so, whatever copyright laws are there I guess. I don't know what the legal implications of using them are but they're frequently used in doom mods with no consequence. If you were working on something commercial, yeah, forget it.

You could always photograph your own hands, "palettize" them to doom's 256 colour palette and try to fit the guns against those photos.

Considering my mod to use these templates like skulltag did,I guess I'm fine.