Images hosted on 8ch expire when they are bumped off the catalog in to the archives. Otherwise, there is no timed expiration that I'm aware of.
The 8chan servers must accommodate nearly 18,000 boards, with almost 2000 posts per hour, many of which will include images, PDFs, SFWs, oekaki pictures, etc.
CodeMonkey would have all the details. admin@8chan.co