I painted a picture on a rock with watercolor then sealed it for outdoors with the same sealer I always use for outdoor things. The sealer has peeled off the picture - not the rock. Is this normal?

It is possible that the sealant you’ve used attached itself to the paint, not the rock as well, or an insufficient amount was on the rock, allowing the peeling to start. Hence, when it peeled, it brought with it what it had attached to, leaving the rock bare. Next time a few things that might help include: Ensure that the rock is both thoroughly cleaned and dried before painting, try acrylic paint instead of watercolor paint and ensure that the sealant used covers the entire rock, both the painted surface and any rock surface not painted. Another option is to match the sealant to the paint type as well as checking it’s 100% suitable for outdoor use.