easy click on the link 'User CP' its on the left near top of the screen next to FAQ and Member list then just select the option from the left hand navi bar.
You can either link (ie the image is on another website) to an image, in the same manor as posting pics or you can upload one.
It gives you the maximum pixel dimensions and/or kb. Use something like Photoshop, Paint Shop Pro or similar to create the correct size image.