Twitter is testing a feature that lets you apply stickers to photos. Similar to what you can do with Facebook, Twitter's image editor lets you add colorful stickers through a new tab. Only some users have reported seeing the feature.
Stickers are being approached differently on Twitter than they have been elsewhere, however. The network includes an option to "see how other users from around the world have edited the same photo." You can also see the photos Twitter recommends you edit in order to add to a discussion.
While you can use the image stickers feature for your own uploads, it appears Twitter is framing it as a way to contribute to network-wide discussions using other people's photos.