I don't know about anyone else but I find those popup onexit windows irritating in the extreme and whenever it happens to me I vow to never go to that website again.
If that is the kind of response you are looking for then be my guest. But I don't think that is your intention. So I'd recommend going first to the real problem then looking for another solution.
If you want people to return to your website you need to:
1) Give then a reason to return to your website.
2) Make sure the reason is something that is actually important to them and of real interest to them.
3) Effectively communicate that you offer whatever that "reason" is.
4) Make sure they have an easy means to return.
5) Don't beat people over the head.
For example, if you update images once a week put a notice on the appropriate page indicating that "Be sure to bookmark this page, we update each Friday, return often for fresh blah blah.."
Then you could hyperlink from bookmark to some easy instructions on how to make bookmarks.
I think this will give you better results.
