I moved my custom icons to c:\garmin and they still did not work for me in 6.15.3. As a matter of fact they quit working in 6.13.7 until I moved them back to the MyDocuments\My Garmin directory.
They have to be now (since 6.15.3) in the same folder than the MapSource installation - in my case c:\programme\garmin. Only there they are working on my computer!
They also changed the transparency color, its not pink anymore. So I transfered my bmp's now in prn's and set the background to transparent...
