Here's an example of the layout I used last year and will use again this year. This is not a Christmas Card (I can't find where I have the photo of mine), but in stead of the "scrabble" part, you can put a 4 x 6 photo. The size of the card stock is 4 x 12 and you score at 3" from one side (that makes the flap to stamp on the outside and write your message on the inside).
Hope this helps.
photo card example