Obvious, but tricky. Take a look at the bug: https://bugzilla.mozilla.org/show_bug.cgi?id=847223

It comes in ten parts, and there's lots of discussion about how to deal with viewports, when to load things in, etc.

The original bug (which discarded images that weren't visible) was even worse: https://bugzilla.mozilla.org/show_bug.cgi?id=689623