Image size and resolution
All images that are longer than 1920 pixels in width or height will be resized to 1920 pixels. To avoid quality loss make sure your images are smaller than 1920 pixels in width and height.
Before uploading your images, make sure they are in sRGB. If you are using Photoshop go to File >> Save for Web. This will make sure that the images will be optimized for web browsers.