Edit - I see you were successful. So you don't need to read this

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That depends on the picture software you use, you need to resize the picture, save it and then upload it to imageshack. It also depends on what OS you use, if Windows (XP, Vista) - then I am not sure if this can be done without some additional software?
There is also one easier way - you will see different links after uploading a picture to imageshack, there are some "thumbnail" links. You can copy the whole link and post it here. But most people don't like this, because it will open a new website with ads..
It would look like this:
If you don't have any software for editing pictures, then I would recommend Irfanview, which is freeware and it's regarded by many as one of the best.
http://www.irfanview.com/ . Just open the picture in Irfanview, choose Image > Resize/Resample. And then Save the edited image and upload it to Imageshack.
Example:
http://img165.imageshack.us/img165/9...board01kj2.jpg
I am sure there are a lot of other different ways how to do it...