Twitter and self-promotion go together like peanut butter and jelly, but without the right tools it takes work. I've recently begun using a handy service called Autopostr that takes most of that work out of the equation, by posting whichever of my Flickr photos I want straight to Twitter with a minimal amount of effort.
On any photo or photos you wish to share with your Twitter buddies you simply add a hash (#) in the title or "autopostr" as a photo tag. The service scans your Flickr account every five minutes, and takes any of those tagged photos and sends them out as new tweets.
Now I wouldn't recommend doing this on more than two or three photos a day since it's a bit spammy, but this system provides a very quick and simple way to promote photos without having to use URL shorteners or sign into your Twitter account. It's also makes it simple to see which photos you've shared to Twitter by sorting out which ones have hashes in the titles or autopostr tags.
Worth noting is that Autopostr won't start sending out your marked photos as tweets until it verifies your information. It also doesn't tap into your Twitter account using the popular Oauth, so you'll have to give it your Twitter username and password for it to be able to post. Creator Tistan Teunissen tells me that's coming soon though.