It is now possible to specify custom images for map markers. Additional section on properties page is added for managing custom map marker images.

On list customization page it is possible to specify a custom marker reference for a location field.
Each location field per list can have its own marker.

Also it is possible to set custom map marker icon per item using API, just the same way as custom color is specified. The unique reference (= code) of custom map marker should be provided in curly braces {} to distinguish from color values.