GroundTruth now supports custom POI icons:

GroundTruthPOI

The small X’s are OSM nodes tagged as "todo=junction". I use these tags to mark junctions on hiking trails which still need to be explored. X’s are not standard icons for Garmin units (as opposed to parking area icon visible on the bottom of the map).

GroundTruth allows specifying icons as part of of rendering rules on a Wiki page:

GroundTruthIconDef

First you specify colors you want to use (up to 255), and then use those colors to specify a pattern (up to 24×24 pixels).

There are still some things I like to implement before releasing the tool to the public. The first goal is to make the tool accessible to non-OSM-ers too, so GroundTruth will include downloading of OSM data for a user-specified area. Hopefully the tool will help attract new enthusiasts for free mapping.