These days most Linux systems use GRUB (Grand Unified Bootloader) to provide the Linux booting experience which in most cases looks very basic as you can see from this Wikipedia screenshot: http://commons.wikimedia.org/wiki/File%3AGRUB_screenshot.png
In order to change the default background of GRUB you can follow the next simple procedure:
1. Select an image file you want to use and save it as a png. For some reason jpg files do not show up correctly in all GRUB versions so use png instead. Note: I tested images which were 1440×900 but other resolutions should also work.
2. When you have the png file place it inside /boot/grub folder. You need sudo or root in order to copy or move an image file inside /boot/grub.
If you use sudo you can do the following in terminal:
sudo mv myimage.png /boot/grub
3. When your image of choice is placed under /boot/grub open up a terminal interface and as a root or sudo type in the following:
and some output should appear which contain the line:
Found background image: myimage.png .
Once you have updated the GRUB configuration you are done. Next reboot the computer to verify the results.