Hi. I've seen in the ArtBox @ the Wiki that the default bootscreen can be changed. How do I do that? And where do I get the bootscreens? Maybe better, what can I use as google searchterm? Because when i try "bootscreen osx os x" I get thousands of results pointing to bootscreens for windows XP.

The white/grey screen with the Apple and the spinning gear icon is embedded in the boot files which are written to the boot sectors of the partition. Because of space limitations, the graphic data is really small. The limitation being the number of sectors defined to hold the boot files.

In a real Mac, I believe the image is enclosed in boot.efi at /usr/standalone/i386, but Hackintoshes don't use this file for boot (notice ".efi").

I have a widescreen and the apple looks squashed. Any idea of how to fix it ?

There is no resolution control during the display of the white/grey Apple screen. You can try booting with "Graphics Mode" = "1280x800" (or whatever) and see if it fixes it, but I doubt it.

Just press F8 at boot and type the Graphics Mode command. If it does help, then add it to the boot.plist as:

<key>Graphics Mode</key>
<string>1280x800</string>

Thank you very much for this answer.
I just thougt that it was possible?
What about the screen after that? With the loading stuff text? Is it possible to change it?

yea
add to boot.plist :

<key>Boot Graphics</key>
<string>Yes</string>