I've been creating custom maps for my Garmin units for many years. They consist primarily of lines and polygons (areas) versus images. For example, I create my own custom maps of Washington hunt area boundaries for GMUs, deer and elk areas, and so forth. When loaded I can instantly see what GMU/Deer/Elk area I am in (or not in.) I've made custom maps for things like the training areas on the Yakima Training Center, for No Shooting zones in King County, etc.
The data is publicly available here:
http://geography.wa.gov/GeospatialPortal/dataDownload.shtmlYou need a few basic tools to convert shapefiles into something that a Garmin GPS can understand. I bought ExpertGPS for around $60 but the rest of the tools needed are free downloads.