I think the MX53 QSB uses the "logo_linux_clut224.ppm" from your "<your_ltib_dir>/rpm/BUILD/linux/drivers/video/logo" directory. You can replace that image with 80x80 ASCII 224 color PPM image, delete the old "logo_linux_clut224.c" and Linux will generate new one on compilation.
If your image is not ASCII and 224 color, Linux kernel compilation will likely fail, but it should tell you what you should do with the image to make it compatible.
I am not sure, but I think you can change the 80x80 resolution of the image too.
I can be more specific, but I am pretty sure that you will find everything you need if you look at the kernel sources under drivers/video/logo
Do not forget to edit include/linux/linux_logo.h and drivers/video/logo/logo.c as well.