I came across lazyload, but the disclaimer at the top worries me.
“Lazy Load is currently not usable. It does not work with the latest browsers as expected. I have currently no time updating the code myself. Patches are always welcome. If you find a solution just fork and send a pull request. Thanks!”
And that was written in September of 2007. That sounds like something to be concerned about. Plus it says it doesn’t work in safari and looking at the comments, there are some Firefox 3.6 issues