This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
If you have a Macintosh with Color QuickDraw in ROM (Mac II
and later machines) get the init DeskPict
Users of compact Macs (Plus's, SE's, and Classics) can pick
up BackDrop instead.
All of these will replace the normal Macintosh desktop pattern with
a picture of your choosing saved in startup screen format. (See the
previous question.) Before saving your picture in startup screen
format be sure to convert it to the default application palette,
or your Mac may display color combinations distorted enough to
induce flashbacks to that Grateful Dead concert in 1976.