A sticker reply might be just a photo reply in a different branding. Stickers tend to be more cartoon-like, and typically are not rectangular and don't have borders. (They use transparency in the image.) Stickers are often limited in availability: they either need to be unlocked by some kind of achievements, or they come in packages that have to be purchased (some of them for free).
- On 2017-06-21, Sebastiaan Andeweg sent a sticker reply to a checkin of Aaron Parecki on Swarm, and then PESOSed it as an reacji to his own site. Aaron's original post shows both the sticker and the reacji.
Facebook Messenger has a downloadable sticker packs that you can then choose from and send in direct messages and in reply to posts. As of 2017-07-31, all packs are free, but the way it is advertised suggests that there are paid packs.
Or used within Facebook Messenger:
Swarm also has the ability to apply a sticker to a particular check-in of yours, when creating a post, like a property of your checkin rather than being a separate post itself. Such checkin stickers sometimes give you extra points (coins) or multiply the points that your checkin earns.
Stickers can be earned through playing the Swarm game. On top of the 100 collectible stickers, there are about 25 extra stickers. The interface notes that "some are only available for a limited time, so keep an eye out!"
Screenshots: stickers can be added to your own checkin, or in reply to someone else's, by clicking on the smiling face next to the input field.