There is no way to edit garmin img files. The one you are doing with MapEdit is called "decompiling". Keep a few things about that:

1. In general "decompiling" garmin img files is illegal! (This is "the real stealing" not unlocking the maps! But this one is a long story and not everyone understand that corretly IMHO...)
2. When decompiling a garmin img map using MapEdit there is loss of information. There is also loss of information when compiling too. So it is becoming mess times two. Just like making a copy of the copy etc.

Suggestion: Try building a transparent map with higher draw priority and use both maps together.

Cheers,