Thank You Pictures
There are two types of thank you pictures. One is usually a nice shot of the bride and groom that can be included with your thank you notes. You don't have to discuss anything in advance with your photographer or do any special preparation to get a shot like that.
The other involves two signs. The bride & groom each hold a sign -- one sign says "Thank" and the other says "You". You can make these yourself by printing letters in a large font and then gluing them to a sheet of posterboard or foamboard, or you can freehand them on the posterboard or have them printed at a printshop. Your photographer may also have signs available for you to use. Usually when you do the thank you signs, they are printed as photo greeting cards, and then you write your thank you note on the reverse. Of course they can also be included with a standard thank you note, or inserted in the special thank you notes that have a space for photos.