Open your file browser and check the option “Show Hidden Files”.

Under Linux, go to /home/user_name/.gimp-2.2. Under Windows, go to c:\Documents and Settings\user_name\.gimp-2.2\.

If the “splashes” directory doesn't exist, create it.

Copy your image(s) into this “splashes” directory. On start, GIMP will read this directory and choose one of the images at random.