This idea is another floating around several kids art blogs.
Basically, you do a water paint painting. We just watered down regular kids poster paint.
Paint your picture and while still wet sprinkle salt all over.
When the painting is dry, shake/wipe off the salt. It will have a bubbly effect over it.
The salt absorbs the water at a faster rate than the paper.
Some my kids did not have much patience for that part. I think they thought it would happen instantly, but the results don't happen for hours (depending how wet your picture is). One child wiped off all their salt too early and it didn't work. So either tell children repeatedly in advance that it isn't instant, or put painting away for a day out of sight :)