First, something changed in the Rest API for retrieving user images. This has happened in the past few days, without warning, unless I missed it.
This bit of the docs located here: https://dev.twitter.com/docs/api/1/get/users/profile_image/%3Ascreen_name
This method must not be used as the image source URL presented to users of your application.
Why not? What is the proposed solution, then, for displaying a user's image in browser? pre-fetching all images then returning the actual URL by reading the headers? That's ridiculous.