We have been diligently improving the image quality for components that use images. New guidelines for the best image display are as follows:
Slideshow Images : 900px – 1000px
Images Component: 400px – 1000px
Moving forward, the best practice for displaying better image quality is to upload the images to the DAM and use the Image Component (formerly Image with Crop) to place them on the page. Recently, Adobe announced that they will eventually sunset the Classic Interface and replace it with TouchUI. By using the Image Component, image optimization will happen automatically and the images will be compatible with the TouchUI.
View the Image Size Guide in Canvas for detailed guidelines.