Transparent maps are usually maps like contour map, geographical grid map, POI map. Generally maps that don't contain area objects. Most probably none of your map is transparent.
Garmin maps in nuvi are files with extension .img, there are some rules about file name and file directory. I'm not sure what is directory structure on 7x5 but you should be able to use at least 3 *.img files with names like gmapprom.img, gmapprom1.img, gmapsupp.img. Each file can contain multiple maps.
It doesn't matter in which file is your map, you can switch on/off each map separately with a menu. It is not recommended to switch on 2 routable maps of the same area. Most probably nuvi will use only one of them. Which map will be used depends on internal map data, like map class and draw order priority.