This means that the thumbnail, GIF, or video still image seems to be a low quality image. Your thumbnail, GIF, or video still image must be sharp, cropped well, not overly discolored, and must have minimum original dimensions of 600px in length and 400px in height. With Best Practice recommendation of 1000px in length and 600px in height
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How To Fix
​Use a high resolution GIF or image in the thumbnail or still video image slot. Do not stretch a small image to the minimum dimensions; this may reduce the image resolution or distort the image.
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To learn more about this rejection reason, see thumbnail technical requirements here and thumbnail best practices here.
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To learn how to fix and resubmit your campaign or campaign item, click here.