21.1% of 26.7M image requests over 5 KB on top-1M pages have file names that say nothing about the picture: 10.5% are hashes or UUIDs, 9.5% are only numbers, 0.84% are camera defaults like IMG_ or DSC_ and 0.21% are screenshot names.

Source: Image File Names on Top 1M Sites: 1 in 5 Describe Nothing - PageSpeed Matters