They are LINE stickers, LINE is a messaging app. There are sticker packs for a lot of series. To download a pack, go to http://dl.stickershop.line.naver.jp/products/0/0/1/???/iphone/[email protected]
and replace the question marks with the pack number. There are three Yuru Yuri packs, the numbers are 5271, 7457, 12647.