Hi, Jeffrey. As a reader, I've had so many frustrating experiences with trying to read maps, etc., I'm thrilled to see an author put serious thought into this.
calibre will convert just fine, but I think it scales images to fit a screen, so I'd watch for that - maybe someone can give us the details of when and how that's controlled.
I would prefer the books I purchase not to have the images down-scaled; that way, I can load up the full size version to see in K4PC.
As for the rest, I can only tell you that the only way I've found to make a lot of images - such as maps and family trees - readable on small devices is to split them.
Orientation... what Kovid said, although, if you can split them, try to keep it in portrait, so they're not neck-breaking to read on K4PC, for example.
The number one thing to do, though, is make sure the images are available online.
I adore Bernard Cornwell, but his images are too small to read, and
he doesn't have them on his site, nor are they elsewhere, and
his wicked publisher actually blocks them from the "Look Inside" features of Google and Amazon, etc. argh.