All the Twitter clients for the iPhone that I've seen make it easy to take a photo and post it to Twitter (via a service like TwitPic). Many of the Twitter clients on the Mac have features for posting photos too, but they often revolve around selecting files, which isn't easy if all your photos are in iPhoto. And it's a bit silly to import your normal digital camera's photos to iPhoto, and then sync them to the iPhone just to post to Twitter.
(Tip: In iPhoto, to view a photo's file in the Finder, Control-click it and choose Show File from the contextual menu that appears. You can then drag the file's icon into an Open dialog to upload it to TwitPic, for instance, but whatever you do, don't move or rename that file!)
Blue Crowbar Software has just come out with another simple solution to this problem: iPhoto2Twitter, an iPhoto export plug-in that posts a selected photo to Twitter via TwitPic.
Once iPhoto2Twitter is installed, select the photo you want to post to Twitter, choose File > Export (Command-Shift-E), click the iPhoto2Twitter button, enter a message, choose an export size, and click the Export button. iPhoto2Twitter posts your photo to TwitPic and the message, with a link to the photo, to Twitter.
Of course, the first time you use iPhoto2Twitter, you must click the Setup button to enter your Twitter login credentials; it can also pull Twitter login credentials from your keychain, making it easy to switch among accounts.
That's really all there is to it - iPhoto2Twitter is a one-trick pony, but if you've avoided posting photos to Twitter because of a lack of integration with iPhoto, or if you just prefer to think about photos when you're already in iPhoto, iPhoto2Twitter is ideal.
iPhoto2Twitter requires Mac OS X 10.5 Leopard and works with iPhoto '08 and iPhoto '09. It costs 4.95 euros and is a 566 KB download. Blue Crowbar Software also offers Aperture2Twitter, which provides the same functionality for Aperture 2 and costs 5.95 euros.