Choose reasonably large thumbnail image bounds, and couple that with HiDPI thumbnails (I finally started just forcing that setting in my skins). There really is no need for thumbnail variants - the largest file sizes you might end up are just fine these days, even on a somewhat bandwidth-limited connection. No one is using dial-up.

And for the same reason, I think slide variants are just a waste of code (ETA: and more to the point, a waste of inodes, which is what's usually limited on shared web hosts). Make 'em big JPG's, and move on. The only situation in which it makes sense to use image variants is when the content of the page might change - on a large monitor, an image of a person with a fair amount of background around him, but on a phone, a more tightly-cropped image of the face. In other words, variant images for artistic reasons, not for file size reasons.

Anyway, I prefer variants over HiDpi images, perhaps just because it's the newer one. However, I must admit the blockiness of the WebP images drives me crazy. And it seems the library's developer is not interested in fixing this. So I always set the 2x variant to JPG. I like to be able to zoom in a bit. And I don't really care about the inodes, because I'm on an unlimited plan.

People with typical hosting plans often discover that "unlimited" doesn't really mean "unlimited." There may be no limit on storage space or bandwidth, but there's usually a limit on inodes. As an old sysadmin, I know why. Those limits usually appear the first time you have to do a disaster recovery from backups, and you find out that restoring a 10GB file takes very little time, but restoring one million 10KB files takes something just this side of forever.

May be David should visit his own shop: this lightbox is already 5 years available in my PhotoSwipe skin!