I have had some trouble with BMP conversions too. I have typically specified
BMP3:outfile.bmp and I normally get 24 bit BMP files. sometimes I get an 8 bit BMP when converting JPEG files. I just tried the
-type truecolor (having seen it here) and now it looks like I get a 24 bit BMP in the cases where I used to get only 8 bits.
I also use these switches:
-units PixelsPerInch -density 72 -compress None -depth 24
-depth 24 should do a 24 bit BMP (the depth -8 in your case may also be causing a problem) but as I said I sometimes get 8 bits even with the 24 specified.
-density cases corrected a problem where sometimes I would get a 0 for the
biYPelsPerMeter fields in the BMP header (it did not seem to cause a problem though). by the way I have not found a good explanation of these 2 fields.
-compress None was required because sometimes a conversion generated a compressed BMP - no logical reason for doing that was found.
hopefully this will be of some help but I am not an expert on this subject - I just tried a few things and looked around using Google and came up with the above switch settings.