Do these comment stickers from Pixiv come from anime?
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.