If you want to create your own icons for your custom POIs, it is possible According to Garmin's help file:

"POI Loader can associate customized bitmaps (.bmp) with the points in a data file if the .bmp has the same file name as the data file. For example, a customized bitmap named "Speed_30.bmp" would be associated with all the the points in the file named "Speed_30.csv" or "Speed_30.gpx." Bitmaps must be saved in the same directory as the associated data files. Your Garmin GPS device may have image size requirements. Most compatible Garmin products work well with icons that are 24x24 pixels or smaller. Bitmaps that do not meet those requirements may appear distorted when viewed on the unit's screen. Custom bitmaps can also include a transparent color. Most compatible Garmin products use magenta (RBG 255, 0, 255) as the transparent color. "