At one point during our day, we went off to have our photo taken. We're going to use an image from that printed for our thank you cards.
Only thing I'd say about trash the dress idea is that it could be months before people get their thank yous. We're still waiting to get our image, and then it'll be a week or so printing and then time to sit and write and address them - so a pic from the wedding itself as a thank you will probably arrive about 5-6 weeks after the event. I'm really conscious of people realising we're grateful!