One trick.
If you have, for example, 3 custom logos and you select random, then one of four logos you will see will be a Bookland logo. I have nothing against Bookland, I even like their logo ;-) , but I do not want them their logo instead of one of my converted Escher graphics.
The remedy is simple. Name one of your logos bookland.bmp
By the way, I have used gimp to scale and/or crop the images to 800x600 or 600x800 size and then I have used Linux commandline binary 'convert' from imagemagick package.
I have cropped and/or scaled pictures manually, because the raw pictures were very different and each of them required different manipulation.
This is an example of how to use the convert binary to produce working bmp file.
convert escher01.png -quantize GRAY +dither -colors 16 -rotate "-90>" escher01.bmp
the -rotate "-90>" is an interesting option, because it rotates the picture only if the width is greater than height.
There is also windows binary of imagemagick available. Imagemagick is the most powerful conversion software for bitmaps you can get. Just have a peek at the options
http://www.imagemagick.org/script/convert.php
By the way, Ulli, bmp format does not store the resolution info, so you can set any resolution you want, as long as the picture is 600x800pixels. 600x800 pixels is the screen size.