GIR save all logo in 24-bit bitmap format for you to edit. When the new bitmap is loaded back into GIR, it will convert to the appropriate format in the .bin file to be used via RGN.
If you read the GIR readme.txt file; it said that it support various image formats based on the type (eg: 10, 11, 12 or 19) The statement "GIR appears to only support 24-bit images" is incorrect. The trick is to find out what GIR type the GPSMap 66 uses. Without the original logo file to look at, I can't really help.
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