Sytist has this function, and I used it to show a before/after for what a proof looks like online vs. what they're receive in the mail after ordering and we retouch. It could also be used to briefly explain a promotion, etc. ImageQuix also has this functionality. I can't find it on either of their sites, but this is a major thing I miss from Sytist.

Make it show once, and have a small link after that says something like "show welcome page again."