There are several ways you can go about it if you want to post a tweet when you can upload a photo to Flickr. The official Flickr way is add Twitter as a blog in your account “Extending Flickr” page and clicking “Blog this” on a new picture you want to tweet.
While this works perfectly well there is also an automatic way if using the Flickr REST API. One of the options is to generate an RSS feed. Using this and the free TwitterFeed service I can automatically tweet whenever I add a new image to Flickr.
The Flickr FSS feed looks like this: http://api.flickr.com/services/feeds/photos_public.gne?id=[[your flickr user id]]&format=rss_200.
Nice and simple. And there are additional option of the RSS feed to filter only images with a specific tag if you don’t want to auto twitter al your new images.