Recommended Image Size for Contact Pictures?

Hi All,

I want to edit up some family members to attach thier images to my iPhone contacts... Does anyone know what the preferred resolution size is for the iPhone for optimal appearance?

I've tried to do this with outlook before, and I hate the way it dithers the images to a low quality.... I'm hoping that the iPhone doesn't do this and so I wanted to experiment with how they would appear...

But instead of making the images just any random size, I wanted to crop them to the preferred image/pixel size so that my photoshopped images look like what I expect them to show up like...

If you could advise with the documented or known pixel resolution size used by the iPhone it would be appreciated!