The equivalent to Imageshack in Twitter is called yfrog. It is designed by the same creators of Imageshack, and it offers you an easy and simple way of uploading images to Twitter.
Its use couldn't be simpler: all you need to do is indicate which image you want to upload, write a message to accompany the image (optional) and proceed to upload and instantly publish the image.
In order to use it, yfrog requires you to have a Twitter account. This offers various advantages like showing an upload history, which can be consulted at any time.
Another very interesting option is the possibility of also automatically publishing images on Facebook and MySpace.
yfrog becomes one of the best options when it comes to uploading images to Twitter.