Checking out some other postcard groups helped me to find the Swap-Bot site. It is designed specifically for snail-mail swaps of many things including postcards. I created an account and signed up for my first swap.
I have decided to create my four postcards with brush and ink on watercolor paper. After visiting the profile page of the person I was assigned to receive my cards, I have decided to create some insect and animal images. My first postcard is of a dragonfly resting on a cattail. I will post the other three images here as well as the four cards I am slated to receive from somebody else participating in the same swap.