I’m not familiar with the OP’s Nuvi, but the Garmin Oregons and Colorados support multiple IMG files in a “Garmin” folder and they can be named human friendly names like Iceland.IMG. When they are connected to a PC via a USB cable they appear as two drive letters, an drive letter representing the unit’s external memory and a drive letter representing a removable chip, if present. Each drive letter can have a Garmin folder. All IMG files in either Garmin folder are recognized. The folder can be accessed by drag and drop.

Might be worth hooking your NUVI to a PC and seeing if the file structure resembles what I’ve described above.