I have the software to do it (GIMP and Nifskope) and the required plugin, but how do I unpack BSAs? Where do I install the file that allows me to do it?

I think you get BSAOpt from Nexus Mods, RG. If it's not there, then use google to find it. Pretty sure a program like that can live anywhere on your drives....

BSA browser is probably the best of them....

http://www.nexusmods.com/skyrim/mods/59553/?

You can also try FOMM (Fallout Mod Manager, very easy to use and search for files but not 100% reliable with Skyrim so extract individual files rather than all of them, or use its handy search feature to make a note of what you're looking for and use a more reliable unpacker once you know what you're looking for)