Based on the info received and my own PB360 testing, I've updated the Copy Cover plugin to send either 8- or 16-colour greyscale bmps to /system/logo based on which PB model Calibre thinks is connected.
- 8-col: PB360 (orig), PB301
- 16-col: PB602/603/902/903 and newer
I don't know whether 8-col is the better option for the PB301, but in the absence of any info posted here, I've decided to leave it the same as it has always been.
FYI...
paola was good enough to beta-test the new release on her PB903. As with my own PB360 there were no problems encountered with greyscale bmps with dpi>96.
Thanks to those who contributed.
Jackie