Spherical texture creator

Does anyone know of a tool that will take an ordinary graphic, and make it sphere map-able?

For instance, a picture of clouds. If I want to map it to the inside of a sphere as my sky, I need a tool that will stretch it first so it will look non stretched in the sphere.

A sky box will not help me because of the way I am doing clouds.

All I really need is to kow what program will do that sort of texture conversion.

Thank you.