Chans allow anonymous users to upload images and historically this leads into a lot of illegal content, especially stuff like child porn, to be uploaded unless you have strict moderation.
That's 4chan. The other big site is 8chan, where users can create their own boards and there are less rules, which makes it much harder to moderate. Then you also have all of the smaller chans which tend to get spammed by bots posting illegal stuff like child porn.
