So I've done absolutely everything required to get a custom portrait to work, but they keep showing up as these weird mottled blue things that hover to the left of the portrait select screen. How do I fix this?

Are these custom portraits you've made, or custom portraits someone else has made and you've downloaded?

If you made them yourself, be sure the color depth (8-bit, 256 colors) is correct, along with the resolution (72 dpi is best, although others will probably work). And double check to make sure they're the right size.

That fixed it, thank you! For some stupid reason I assumed all .bmps were 256 colors by default.

Great to hear. I'm glad it worked out.

Does one need to add the HUM5_b to userport.mes as well?

You have to add something to the userport file. It's been a few years since I actually did it myself, so I'm a little fuzzy on the details, but I think you have it right.

The easiest way for you to see how it's done is to download one of the custom portrait packs others have posted, and examine the included files. That's how I figured it out.