Originally there was the option to create a square card with MyDoodle but now cards are all rectangular (either portrait or landscape).
You can resize square designs so they will fit the card using the creation tools available in MyDoodle.
This does leave you with quite a lot of white background which you could use to create a personal greeting. I am gradually adapting my square designs so they fit in better on a rectangular card.
Here are the most recent adaptations.
You may be also interested that MyDoodle recently featured an interview with me for their blog which is posted here