I just checked my copy of Avernum 1, which was bought directly from Spiderweb back in the day (digital download). I also checked my copy of Avernum 2 which was bought as a CD (back when those were still cool). You can in fact open up the "paper doll" images in Avernum 2 in the DATA folder (at least on my cool, old-school version) - I assume this is the same in the GOG version as well.

...In contrast, I DID NOT find these images in Avernum 1. I have to assume these are hard-coded into the application, or something like that. 1999 will do that to you. If you were intent on modding your character images, well, sorry about that. If you just want the character images for your personal use open up the Avernum 2 or 3 DATA folder and you ought to find what you're looking for.