The best way to get new versions of those images is to extract them directly from the game's data files. Fortunately I've written a tutorial about that: User blog:ZeroOne/Extracting images from Beyond Earth. The title says "Beyond Earth", but that game's data format is identical with Civ5's, so the same instructions will work. Could you please give it a try? I'd like it if someone could validate that my post actually covers the topic in enough detail. :)

Aha! So while that probably would have worked with vanilla units, they have no border. But these are from the Gods and Kings expansion, so they are probably hiding in a different file somewhere, which I can't find.

Try the Expansion2UITextures.fpk file and in there the unitportraits256_expansion.dds and unitportraits256_expansion-index.dds pair of resources? :)

Status update: that worked, but the borders need to be found now. Right now the images I have are borderless, so if anyone knows the resource location, please post in this thread ^^