Often in the preview size it's unclear if an image is computergenerated or proper sharp in specific areas or if the quality is enough for large printing sizes (color noise). So a "zoom in" is needed! I often bought an image an the It was not usable … That's annoyig.
The quality should be always high, but they aren't … For big size quality the image size is not only crucial. And yes, I can complain, but that's laborious especially if I designed with Adobe Stock previews and my client did a release for that and then (after licencing) I have to say "sorry the image is not good as I thought" and we have to design new. That's bad. But a zoom in could be a very easy way to avaluate the image much better. I think fotolia had this?! Or an enlargement like istock, shutterstock etc. would be helpful for layoutig with previews at least!!!