Credit to Pixelfx.org because I didn't know how to do this until I read their tutorial! (Note: I wrote up this tutorial in my own words but all credit for it goes to Pixelfx) This is a pretty cool effect though. You'll want to start out with a white background. MAke the image whatever size you want. I'm going with 200x200. Get your paintbrush tool and use whatever settings you want. I'm using: size-25, step-10, opacity-100, hardness-50. Choose went ever colors you want and put a bunch of dots on your white background. Like this

Now go up to "effects", then to "distortion effects", and lastly to "pixelate". You'll want to have it symetric so check the little box that says that. Then you'll want to choose a number between 15 and 20. Whichever site blocks you'd like in that number range. Click ok and then you should end up with something like this

Now go back to "effects" but this time you'll want to go down to "geometric effects" and click on "circle". Set your background color and click ok. You'll end up with something like this

And you're done! Have fun with it :)