UPDATED 6: 4chan’s current gameplan evolving over at Encyclopedia Dramatica (Great stuff!). Also, from reports gathered on Reddit, it seems this block allegedly is because of massive DDoS-attacks against img.4chan.org.
He claims that img.4chan.org is in fact blacklisted by AT&T because AT&T and it's customers received a huge number of DDOS and other attacks from 4chan. He didn't know if this was a permanent ban or not.
The best about 4chan.org is that there are dedicated boards for each topic in which most popular are music, photography, video games and various others. You can also use the services of 4chan.org without creating any account.