Take a look at the CEPC bootloader disk images for CE 6.0 (C:\WINCE600\PLATFORM\CEPC\SRC\BOOTLOADER\BIOSLOADER\DISKIMAGES\BOOTDISK) - you will notice a file called splash.bmx which would appear to be some form of bitmap file but can't be viewed in the Windows Explorer - so how do you view the bmx file, or a better question would be how to replace the existing .bmx file with your own bitmap.
Windows CE 6.0 Platform Builder tool ships with a command line tool called bincompress that gives you the ability to compress/decompress bitmap files to/from the bmx format.
Here's how the command line looks...
Compresses/Decompresses BIN images to be used with BIOS loader
Usage: bincompress /C|/D infile outfile
/C compresses <infile> and writes the results to <outfile>
/D decompressed <infile> and writes the results to <outfile>
So, you could use bincompress to decompress the existing splash.bmx file like so...
C:\WINCE600\PUBLIC\COMMON\OAK\BIN\I386>bincompress /d C:\WINCE600\PLATFORM\CEPC\
Decompressing : 100%
Input file size : 4582 bytes
Output file size: 10678 bytes
and here's the output file "foo.bmp"
Of course you can also use bincompress to swap out the Windows CE logo with your own custom logo...