Here is a complete step by step guide for resizing images.

http://www.marksandel.com/PicResize.pdf
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There are several ways how you can add images - this is from the FAQ link above:

How do I attach a file to a post?

To attach a file to your post, you need to be using the main 'New Post' or 'New Thread' page and not 'Quick Reply'. To use the main 'New Post' page, click the 'Post Reply' button in the relevant thread. ...