I have a custom code for 900 pixels wide since that's the standard used for most computers, tablets, and phones. When I upload, I use a full 1024 pixels and then the thumbnails are generated at specific sizes to fit the website. While it's not considered "Retina" display capable, it gets pretty close. If I wanted to go full retina, it requires larger initial images with a custom css for displaying Retina images. Everything scales to the device you're viewing it on.