Yeah, agreed, it's probably better handled in reverse.
Ideally, you'd just click once and it would toggle (Polycount style), but I'm not sure how to accomplish that in css.
I believe :focus is what you're looking for (well, not exactly but it'd be more usable at least), but I don't think that works for img =/ Maybe some simple onclick javascript event that modifies the CSS would be easier.
EDIT: apparently :active works on any element on modern browsers (your only issue would be supporting IE below version 8, if you care about that, and this doesn't mention if this works on Safari)
https://developer.mozilla.org/en-US/docs/Web/CSS/:active#Browser_compatibility