When I made mine, I just did a search for "Disney clip art" on google and found the characters I wanted. Then I just copy and pasted the clipart into a Word file and printed that on the envelope. I also typed "Thank you Mousekeeping" in Disney font (you can download this too). It came out fine. You just have to play a little bit to get the pictures to print out in the correct location on the envelope.
The year we went the week after Thanksgiving I used the characters dressed up in Christmas attire, etc. The year we went the week before Thanksgiving I used the characters dressed in their Thanksgiving/Pilgrim outfits.
Maggie