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It is raster imagery. That's a fancy way of saying "pictures".

True, but to elaborate, the "pictures" will be georeferenced so that the GPSr can show your position accurately with reference to the imagery, so the imagery and the vector map will occupy the same place on the ellipsoid we call "The Earth".

It's a bit tedious but you can use any .jpg imagery by georeferencing it to the map that is displayed.  You have to make the image semi-transparent then match features on the image to the same area on Google Earth, stretching it until they overlay properly.  Once that's done you save it and send it to the GPSr as a "custom map".

I use it for trail maps that are provided by organizations so that I can see things like traiheads, markers, etc.