Any image that's been coded with height/width stats, anyway. fewer people post images with the data defined these days, modern browsers find it less useful and decent CSS structuring works around it anyway, so it's rare you see it actually work, but if someone does code properly it can block large, defined, images only. But it defaults to assuming any image undefined is a large one and blocking it.

Arguably, we could do witha better image posting interface that either prefetches or asks for the dimensions and codes them to modern standards.