In the Open-dialog you get when you click New Image Resource, type *.png and press enter. Then you filter the "view" so that only PNG images show. You can use this trick to add other file-types as well.
I've used this trick in other applications for years, and thought it was common knowledge. A colleague of mine wasn't aware of this the other day, so I thought I should share it with you as well.
I don't know if this works on Mac/in Linux as I'm not familiar with their file system browsers.