Update (21-Aug-2011): This service is not available any more.
TwitterGram is an interesting free service for Twitter. It lets you post your Flickr photos directly to Twitter. You can choose to post all the photos to Twitter, or only photos with a specific tag and TwitterGram will take care of the rest. The best part is that TwitterGram is completely free, and totally online, so you don’t need to download or install anything.
To start posting your Flickr photos to Twitter, just provide your Flickr username to TwitterGram. Then provide your Twitter login details (these will be used to post the Flickr photos to your Twitter account). Then, you can specify whether you want to post all photos to Twitter, or just the photos with a specific tag. That’s it – you are done!
Now, TwitterGram will keep track of your Flickr account and post all the photos to your Twitter account. This is very easy to use, and works like a charm. Try TwitterGram.