Stretch my Wallpapers, Please Help Me

[Sage's Unicorn]

Hi;​

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Recently some of my wall papers seem to have shrunk and try as I might I can not seem to get a little arrow at the side to stretch them out. I thought I had seen someplace where you can adjust them in general to stretch to fit but I can not seem to find that. Can anyone please tell me if there is such an adjustment, now I am beginning to wonder about my memory! And if there is such a thing can you please tell me where it is located?
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