Adding a decal to sphere:
In my previous tip I stated you should use an image that is twice as wide as it is high. This holds true for adding a decal to a sphere. Below is the image I used to add a decal to a sphere. It is 300 x 150 pixels.
The blue area was masked in a paint program and converted to an Alpha Channel. Then Spherical Texture mapped to the yellow sphere, it leaves only the circle area visible on the sphere as seen below.
Before
After

 

 

 

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