Taken from the openoffice.org forums:
The way to set an image background for an individual slide is to choose it using the background tab on the Format | Page menu for the slide you want to add it to. The complications start here. If you want a custom color or bitmap background, you can't do it from here. Close the page menu and choose Format | Area. From there you choose the type of background you want, then add it to the list (For custom colors type in a color name, then "add", then edit the color to what you want it to be. For bitmaps, use import.) This will put your graphic, color or gradient on the program list. Now go to the Format | Page menu and choose the background you want. Your color or bitmap will be on the list.