These maps are designed as spherical maps.
The image to the left is an image as you download it from this site.
You have two ways of approaching the use of these maps.
You must instruct your application to wrap the image 1X the width of the Globe or hemishpere.
The you must have the application to wrap the image 2X height of the globe.
the plan is to get the bottom of the image (the base of the clouds) to sit right on the equator of your 'sky' globe.
You can take the image into an image editing application.
First, double the height of the image
Second, Fill the lower half with some color (I have used black in this example).
Now you have a full height image that can be mapped onto your sphere with a Width wrap of 1 and a height wrap of 1.
If you simply map the image provided as is onto your globe, it will distort and you will not get the correct effect.
The above image shows some mapping controls from Lightwave