My prefer way of "thumbnailing" them is to let Finder do it themselves to keep the consistency.
First, change the file format from Photoshop to Preview.app and make sure that Finder is set to give Icon a preview (CMD + J). With that, you should be able to view all pictures on the icons.
I dislike Photoshop's preview icon. If the image is 2x2, the icon will be set to 2x2, making it very hard to select. Bad idea.