how the heck do you stretch the pic horizontally

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how do i make my picture bigger horizontally, i know how to use the more space feature to show the picture more vertically, but i would like for my picture to stretch from left to right on the top of the screen, just like regular firefox personas do, the whole reason why i downloaded this add on in the first place was so i could bring the screen down and so i could use my own pictures, but there has to or should be a way to stretch the pictures.....