I wrote "thank you for the generous gift." I read some etiquette things that suggest saying what you're going to spend the money on but that just sounded awkward to me
I already sent out thank you cards after getting any gift for engagement parties, shower, etc. so I just had the wedding ones to give out last month. The wedding picture Shutterfly ones just went to the people that came to the wedding in Mexico. Other people I sent more generic thank you cards I picked up at Homesense, Target, etc.