The ban is in place due to some viral photographs of a gas explosion in Beijing which were hosted on Twitpic.
If you are looking to access Twitpic in China you can make use of any of these free proxy servers.
Another solution is to use Freedom stick which allows you to browse the internet anonymously using Tor.
In addition to that you can also read our earlier guides on browsing the internet anonymously.
If you have any other methods for this please feel free to share it in the comments.