Per their documentation:
The icons must respect the following technical constraints:
Square aspect ratio (1:1)
Preferably 512?512 image size (or 256?256 if not possible)
PNG format with alpha channel
Size must not exceed 1 MB
Image should look good on a white background and on a black background as well (to support both app themes)
but if you don't find any you like, its really easy to snap a pic with your phone and use an online photo editor
http://apps.pixlr.com/editor/ to adjust image size and storage size to their specs. More often I'm doing it to replace the background image to match things like photo of rooms, lights, fans, pic of house with exterior lights on for outdoor lights, etc.