Maybe I miscommunicated. The thumbnails on the sites I am referring to (most all sites) are used as links to larger images of the entire work of art. By vignettes, do you mean crops, and if so what size? If the size of thumbnails, yes, I often see this. If larger crops as the only representation of a work of art, I don't see that much except as an intro to the site.

Really, it's all over the place and likely changing, too. Earlier in this decade, most gallery sites would not post an image larger than a few inches (forgot but about 300 to 400px or so widest dimension). I think the reason was to prevent copies from being made and used in a variety of ways without permission from the artist(s). Who knows. Some sites still do this conservative bit.

Gradually that rule was broken, and sometimes the pics are huge. Some sites have pics that enlarge almost filling the monitor. The MOMA has had some inventive designs for a few exhibitions. But that's MOMA.