Yes, on newer models (like 37xx), only 256-color bitmaps are visible for the map symbol. Anything else is not visible. I have to redo my tests, but on my 760 all formats were working.

For the Where-To icon, I would use bitmap sizes which are a multiple of 4, like 48x48, 40x40, ... On my 760, 41x41 did not show up. And yes, you have to specify the path to the Where-To icon in the Preferences. In fact, you need 2 icons: MRT1.bmp and MRP2.bmp. The first is the normal icon. The second is the pressed icon.