This is a tutorial on how to create a custom Garmin map by using google earth to create a KMZ file with an image overlay. Then take the KMZ file and drop it into your CustomMaps folder on your Garmin device. In this example, I am using a GPSMAP 62s.
Compatible models are: Dakota Oregon Colorado GPSMAP 62 series GPSMAP 78 series eTrex 20, 30
Other notes: the size of the image overlay should be up to 2000x2000, otherwise you will have problem with google earth. Also resize the images to 155 dpi if they are higher. Don't need to increase from 72dpi up, as that won't make a difference and only will increase file size.