When you plan to photo texture for a sky prop, use the 2D Face primitive. By default it measures 20.48 X 20.48, a square. To make the photo fit perfectly you need to know the aspect ratio of the photo. Dividing the width of the image by its height gets the aspect ratio. For instance, 800/600=1.33, 640/480=1.33, 300/200=1.5 and so on. This information you can get from most any image editor or viewer. You can also get this information when you import the photo in the 2D picture editor by right mouse clicking and hitting properties then the summary tab you should see the images specs including its width and height. The 2D Face should have the same aspect ratio as the photo in order for the photo to fit squarely (no pun intended) on the Face. So if you multiply the x-axis (width) of the 2D Face by the aspect ratio and put that number in its attributes window, you then have the correct aspect ratio. So for example if we have a sky photo that measures 756 x 486, dividing 756 by 486 we get an aspect ratio of 1.56 (rounding to the nearest figure) and 1.56 multiplied by 20.48 (default 2D Face) gives 31.95 for our 2D Faces x-axis. Our photo will now fit without tiling. We can now rescale the 2D Face uniformly using the rescale feature in the editing menu.