Do these comment stickers from Pixiv come from anime? In Pixiv, you can leave a comment on any fan-art. There is also an option to leave a sticker for comment. They include an elephant, fox (?), girls, and skull. 



Here is a fan-art example from one of those stickers. I've tried to google the tag but couldn't find anything.



I'm asking about all the stickers - elephant, fox (?), girls, and skull - , but more specifically, the one for these girls above. Do these stickers come from an anime? If so, which one? If they are original characters, do they have any names?
My guess is that the stickers do not come from anime, although I agree they certainly have an anime "feel". A professional site would be very foolish to use copyrighted material without permission.

Possible sources for the artwork would be from their own staff (including contractors and subscribers), public domain image repositories, and sites such as Deviant Art. For the latter, they would need permission from the individual artists, but many of them would be happy to give it in exchange for credit somewhere at the Pixiv site.