Photoshop? or...?

[Bogru]

Which software is this guy using to create his pictures?
Eric Daigh

This is what it said in an article somebody wrote about him:
"he uses a computer and specialized software to break an image down to a very low resolution and force the computer to make the image out of only five colors, the primary colors of red, blue and yellow, plus black and white."

If it's Photoshop, How do he do that in Photoshop?

Thanks