Okay, so after I posted the entry yesterday, I remembered that I'm in a FB group where people can go to ask for help when making covers. I posted this image and said "Hey, how do I do something like this?" and they were very helpful.
Mostly I just needed them to tell me what words to search on, and so I can figure this out the next time I want to do so: bevel. There was also some discussion about lighting which went way over my head (which may be a reason for me to pay someone else to do this, but... whatever).
Anyhow, this is what I finally did in GIMP:
- On the layer with just the caduceus, Filters->Decor->Add Bevel...
- Save the bumpmap
- Then activate the layer to make bumpy, and Filters->Map->Bumpmap... and click on the Aux Input section.