• create a twitterfeed account
• upload a photo to flickr
• tag it with ‘post:twitter’ (or something more imaginative)
• grab the url for the tag feed (you can get this by clicking on the tag and copying the url from the ‘Latest’ feed link near the bottom of the page
• create a new twitter feed using the url from above
• set the frequency to whatever suits

Et voila, twitterfeed will check your flickr account for new photos with the ‘post:twitter’ tag (or what ever tag you used) however often you set the frequency for and it will dump a message into your twitter account with a link back to the photo and any extra text you specified.

Groovy

Calvin: “Ah, I got the letter I wrote to myself.”

Hobbes: “What did you write?”

Calvin: “Dear Calvin — Hi! I’m writing this on Monday. What day is it now? How are things going? Your pal. Calvin.”

Calvin: “My past self is corresponding with my future self.”

Hobbes: “Too bad you can’t write back.”

Calvin (next day): “I got a letter from my past self.”

Hobbes: “What’s it say?”

Calvin: “Dear Future Calvin — I wrote this several days before you will receive it. You’ve done things I haven’t done. You’ve seen things I haven’t seen. You know things I don’t know. You lucky dog. Your pal. Calvin.”

Calvin (sniffing): “I feel so sorry for myself two days ago.”

Hobbes: “Poor him. He wasn’t you.