(h) means hashed
(u) mean unixtime stamped
unixtime is the number of seconds elapsed since 1970-01-01 (in UTC)
hashed filenames are the sha256 hashes of the image file uploaded by the poster
So they are not random
It's easy to test: next time you upload an image, check it's sha256 hash (for example at https://emn178.github.io/online-tools/sha256_checksum.html)
8kun will rename & store your image as [sha256hash].[extension]