@Turbo -- Thanks for the reply.

My logo files were conforming to the above format (I read your post in the other forum regarding the format, and followed it). GIR_Editor could load, save, and replace without problems.

I posted the two logo files in [Only registered and activated users can see links. ]. To recoup:

[1] single 480x272. Splash page is fine, and is replaceable. USB page looks garbled like the splash displayed with wrong dimensions.
[2] double 480x272. Splash is fine (first image), not not replaceable (GPS hangs). USB page also looks garbled like splash in wrong dimension, but was with a very dark tint and barely noticible (i.e. practically just a dark screen). By playing with the images I am sure the garbled USB screen was derived from the splash.

Note that in the above single image version my header did not have the 12 bytes from 0x10 to 0x1B (that gives the number of images). The double image version does have the bytes.

Since then I did some more experiments:
* single image, but with the 12 bytes (setting number of images to 1)
* double images, with the second one 480x212 (i.e. smaller)
Neither would display the splash at all (the splash screen was just black, and the USB screen is blank with default background).

I also tried changing the number of padding bytes at the end (but always rounded to 16). GIR_Editor seems immune to that; and it made no difference to the behavior on the GPS.

At this moment, my hypothesis is that the logo file of the 660 (and cousins) is different from that of the others (with 480x212 logos); it uses a single 480x272 splash image, but somewhere in the logo data there is a parameter specifying how the USB screen should be derived from the splash -- that is what I am missing.

Sean