Signal Messenger supports custom stickers. Awesome!
I recently found out that Signal supports stickers created by users and was very happy about it. For me, the easiest way to install was this site. Thank you, Signal developers!
You can remove them. Tap the sticker icon on the right side of the text entry field (bottom of the screen). Scroll the bottom stickers' bar to the right until you see the "+". Tap it, et voila - you can remove them.