Alright, I've tried it as you said, and sure enough, the B2/W2 version works perfectly on PKHeX now. I'm also unsure why GIMP causes it to not import correctly, but as you said, opening it on paint and just saving worked. Thanks for the help.
Yes, I'm aware of that, what I'm asking is how I can convert a png/bmp to a working psk? I also want to edit the skin to fit the B2/W2 C-Gear, but I can't really import it if I don't know how to convert it to psk. Here's the B2/W2 version:
Elesa 2 C-Gear B2W2.bmp