The Apple TV unit allows you to view most any multimedia file on your television, using network access, external hard drives, flash disks or a computer. Displaying the content correctly can require some adjustments to ensure the Apple TV device receives all the information it needs to correctly display an image.
Load the photo that is not displaying correctly into iPhoto or other image manipulation software on your computer.
Click the "Photo" menu in iPhoto, and select "Revert to Original" to reset the photo to its original rotation and dimensions. Some digital cameras embed rotational information into a photo, and it's this data that causes Apple TV to display the image incorrectly. If you use software other than iPhoto, check its help information for details on how to reset the photo and remove the camera's embedded data.
Save the photo, and resynchronize it to Apple TV using iTunes.