@caspertone
If the above post means you're wanting to split Garmin (proprietary) IMG files there is no known way to do that and re-compile into a properly-working IMG after making changes. Garmin protects their official mapping from complete reverse-engineering. GMapTool can split Garmin tiles and make a simple preview map for use in MapSource but it will not have search functions enabled so if you make another IMG map from it it'll be quite useless in a GPSr.
Spoiler: Link
http://gmaptool.eu/en/content/gmaptool