Grr... Swap Images

Spent about an hour working on this one, but to no avail. Not quite sure where I went wrong. The tutorial wasn't the easiest for me to follow, but I've read worse. The technique looks cool (here's an example), and I think I just forgot to do something small. I might try again tomorrow--though this technique is nowhere near standards compliant. Come to think of it, I know I came across a CSS-only, completely standards compliant trick like this the other day. Now I'll have to plow through my del.icio.us links... So, again, no image to post today.

Just testing a theory: