In lieu of store-bought thank you cards, we've had the kids write notes of gratitude on the back of a picture print for the past couple of years. We either include a picture of them at their birthday party or a posed pic of them, tying in the theme of the party.
We just order them as 4x6 prints, so it's really inexpensive.
This particular pic was a thank you note from an 8 year old painting party.
We had her hold the brush up like she was painting the letters and then, in Photoshop, we used a paint stroke font and changed each letter to a different color. Lastly, we added a little "paint" on the edge of the paint brush.