To upload images to a stack, you can just drag and drop the files on the browser window.
Alternately, files can be uploaded using the Add Files (“+”) button next to the stack title.
For flexibility it is the best if you provide the images in a high resolution. This will ensure your output file is always in the best possible quality. The maximum allowed output image size is 4096×4096 pixels or 2048×2048 when using @2x, which allows you to request images for retina devices.