Okay, nevermind, I figured it out. There are a few kinks, so let me
describe it:
I was using Terraserver to get USGS topo maps, and Mobile Atlas
Creator exports Terraserver tiles under the .png format. Renaming the
Terraserver folder to USGS doesn't work because your program
recognizes USGS tiles as .jpg files... So, short-term workaround, I
just renamed the exported folder as Osmarender, since they both use
the .png format. This is purely short-term, offline use, so it doesn't
matter for the time being, but this could be an issue later on.
Cheers,
M