I may be a little biased because these are my very own thank you's I used for my wedding ... but I do love the simplicity of them. I purchased the rights to my images from my photographer which was a major bonus because it allowed me to use one of the photos and create a thank you card.
I wanted something that my guests could put up on their fridge, or display on a bulletin board somewhere in the house or office. All the while trying to be cost conscious ... I thought that a 5 x 7 postcard style thank you was the perfect option! However, I didn't want it to get all roughed up in the mail ... so I placed it in an envelope to protect it. This also allows for a lot more space for your personal note on the back when it doesn't have to house the address as well.
This worked out great and was able to tie it all together with previous stationary from my wedding day.
Below are two options I used for my thank you ...
The image below merely shows how you can write your personal message on the back of the postcard.