For anyone who might find these useful here is a set of PS CS2 brushes and the accompanying extracted PNG files.
They all come from hi-res photos and are about 10X times the size you need for a Sims eye but they resize without any distortion.
Here are samples images of eyes that were made very simply in Photoshop using just two layers. The bottom layer is black and the color layer was screened or multiplied or darkened on top of the black. Then the the dodging and burning brushes were used. Each eye took about 30 seconds to make.

Here is also a sample of one of these easy eyes dropped into the game.

Hope you find this helpful in the creation process.