After examining the portraits for Avernums 4-6, I think the original portraits somehow got "squashed" between the time they left the artist's hands and the time Jeff got them. Compare the images below:
At left is an example of one of the original portraits as they appear in AV5; at right is a version I stretched vertically a tad in a graphics program. At least to me the stretched version looks marginally better...
If this is in fact the case, I'm surprised nobody noticed/commented upon it before now. :shrug:
Some of the newer sprites (such as G230, G231, G242, G243 in AV6) probably don't need any fixing. Some of the sprites (such as G226 in AV6) could probably benefit from having the modification applied *twice*.