You can share one photo at a time on Twitter directly from iPhoto.
To share, you must be signed in to your Twitter account and using OS X Mountain Lion v10.8 or later.
Open System Preferences, and click Internet Accounts.
Click Twitter.
Type your Twitter user name and password, and click Next.
In iPhoto, select a photo to share.
Click the Share button in the toolbar, and click Twitter.
A Twitter form appears with the selected photo attached.
Type a message up to 119 characters long, and click Send.
Note: The photo accounts for 21 characters of the image URL.
A message appears to let you know that your photo was published.
A link to your shared photo appears in your Twitter feed.