Depending on exactly what you mean by "bootsplash", you need one of two places:
For the small logo that comes up on framebuffer screens, see
http://registry.gimp.org/plugin?id=376 That should also have some info about where the file has to go, and what other steps are involved. (I haven't ever done it myself, so this is in the "hope it helps" category. I'm not certain, but I believe you need to re-compile the kernel to change that logo (it's not a file displayed
by the kernel, it's actually something
in the kernel).
If you want a full-page background, see
http://www.bootsplash.org/ The front page says you'll need to patch the kernel, which definitely means you'll need to recompile it. (The reason these have to be compiled into the kernel is that they're being displayed before any filesystems have been opened up. If the kernel doesn't have it internally, it hasn't got it a all, at that stage.
And for KDE, first answer I found was right here:
http://www.linuxquestions.org/questions/history/32810
If you want to save yourself some time, I found that info by googling for
linux boot splash
linux boot logo (after the "splash" stuff didn't show me anything about the small picture I'm used to seeing, and one of the pages from the "splash" stuff mentioned "boot logo".)
and
kde splash
Hope this helps,
CHL