Steve Outing over at E-Media Tidbits has suggested a "better technique for enlarge-this Web photos". I agree that displaying "enlarge this photo" links usefully is an unsolved problem, but I don't know if the technique pointed out there -- using mouseovers to expand photos -- is the best way to do it. It's a good idea in theory but could get disruptive in practice.
The problem is mouseovers get in the way. For example, if I'm moving my mouse cursor from one end of a page to the other -- say, to go up to the navigation bar -- my cursor crosses every part of the page between here and there. And if any parts of the page are set to trigger onmouseover events, they're triggered, despite my not wanting to trigger them.
That exact scenario happens almost every time I visit CJOnline.com, which relies heavily on mouseovers to dynamically change content on its home page. When it doesn't happen, it's because I have carefully considered the path my cursor should take to get from one area of the page to another -- an experience not unlike avoiding minefields. (Disclaimer: CJOnline is a competitor of the site I work for.)
The crux: With a mouseover image, you can never tell whether the reader explicitly wanted to mouse over it, or whether his or her cursor was simply en route to another part of the page. That's why the click makes more sense -- the act of clicking signifies intent to do something.
Of course, I'm basing this on my own Internet usage and observation of friends and colleagues -- not on any sort of study.