Twitter explained in a help page "User galleries on Twitter aggregate the images you've uploaded in your Tweets into an organized page where you can view all of your most recent images." .
It added "The images included in user galleries can come from Twitter, yFrog, TwitPic, Instagram and other image sharing services supported in Twitter's details pane." .
You'll be able to see a link to the gallery and the latest photos in the right pane on a user's profile.
Clicking on the link will lead you to a dedicated page with the 100 latest photos in the user's stream regardless of where they are hosted as long as the source is supported by Twitter's media preview feature.
You can also check out any of the individual photos in more detail by clicking on any of the thumbnails.
It's a rather basic feature, yet it's taken Twitter so long to build it. Still it's part of Twitter's recent focus on expanding the service and adding new features.
Very recently Twitter added functionality for uploading photos directly to the service. Until now users had to rely on third-party services for this.
For many years the Twitter experience was completed with the help of various third-party services from URL shorteners to photo and video hosts.
But Twitter is finally rounding out the built-in functionality mostly to the detriment of external services. The new photo galleries are the latest example but they're hardly the last expect Twitter to continue to move into new directions.