Twitter limits tweets to 140 characters. When links and/or images are included they too count towards the 140 character limit. An example from Twitter's website:
The character count breakdown is as follows:
An “http://” link = 22 characters
An “https://” link = 23 characters
An image = 23 characters
Example: A submission with an image and an “https://” link will reduce the tweet length by 46 characters (23+23) leaving only 94 characters for the tweet itself. When sending a submission such as this make sure to leave 46 characters as per below.
Note: The character count includes the Promo URL and any links in the tweet itself (Status Text). The Post Title and Author are not considered part of the tweet and do not post to Twitter.
A FAILED status will occur if the submission is over the 140 character limit because Twitter will reject the tweet.
Unsure if your post meets these guidelines? Log into your Twitter account and run a test. You'll see that as you add images and links, Twitter adjusts the count accordingly.
Additional: This character limitation also applies to Plurk and Stocktwits.