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A camera records whether you held your camera horizontally or vertically, when you take pictures and records this information in the image. When you open image in an image viewer and tell it to rotate an image, some images viewers rotate the image, but don't bother updating the orientation information in the image, so when you upload such rotated image to Flickr, it follows the instructions that are still in the image and rotates it, so you end up with an incorrectly rotated image.Review which image viewer or image editing software you might have used that might do that.