Can anyone help me resize a picture into a banner without distorting it?

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Hi, Im trying to make a banner..well a rectangle that goes across the top of a webpage (never done it before). But the picture that i would like to use for it is in the shape of a rectangle that goes down the page. I have paint, pagebreeze and photoscape (havent got my head properly around that one yet) and everytime Ive tried to resize it, it causes major distortion...does anyone know how i can achieve this? Or should I just go for an easier picture to try?