1) Make a new document, anysize doesn't matter. Press CNTRL+D to reset your pallete colors. Set your background to White or any color that suits you best.
2) Open up your graphic you wish to turn into a ball. It is best to use a filter effect, for example I used the typhoon effect for this ball.
3) Select the Eliptical Marque Tool and hold shift to make a perfect round selection like below:
4) Right click the layer and set these blending options:
5) There ya go, cant get any more simpler then this:
More Tutorials: 
Not that one. It's a marble on some heavy, hand-pressed paper.
What has happened to the background? Is it still a planet or what?
How's this?
http://www.geocities.com/itisillegaltoshowernakedhere/cloudymarble.jpg
It took forever to get the color swirls just right.
Nice work btmaisel!
What will be the next step?
Here's taking it a step further and making a planetary closeup:
http://www.geocities.com/itisillegaltoshowernakedhere/planetgreen.jpg