I believe that there are at least two ways to do this.
The kernel has an option to show a bitmap image at the top of the screen, known as a boot logo. Recompile the kernel, and select the option for a VESA VGA graphics console (under “Console drivers” -> “Frame-Buffer support”).
You can also use a full-screen image, but I'm not entirely sure how.
You may also need to patch the kernel for DSL; see
http://www.bootsplash.org/