The dialog is really awesome! Just a few minor design tweaks:

 * The icon in the add button seems misaligned

 * It would be better if the window was wider. The narrowness doesn't have the best feel.

 * Could the horizontal and vertical padding between the images be even, so it's a grid?

 * The vertical padding between the bottom of the avatar picture at the top should be the same as the distance between the close button and the list at the bottom.

 * The layout of the picture and contact name at the top should match the layout we have elsewhere in Contacts - it would be better if the name was a bit bigger or the picture a bit smaller. The name could also be dropped down slightly.

These are all nit picks though. It's really great!