Copilot's image processing pipeline is unique because of its integration of aspect ratios and editorial crops.
On Performance, DPI, Retina displays, and the Web
In general, page-level performance for images depend largely on file size. Copilot's dynamic image server, Vulcan, does a lot to optimize for speed while maintaining quality, but it works best under certain scenarios.
As a note, settings like "DPI" or "PPI" are not relevant in the digital world, they are settings only relevant in print. The more important things to remember are pixel dimensions (the more pixels, the better quality), using the right file type (JPEGs), and color profile (sRGB).
Devices with super-sharp displays like iPads and iPhones work best by using large pixel dimensions, not large DPI settings.
Guidelines
For best performance (both speed and quality), follow these guidelines:
Try to use JPEGs instead of PNGs. JPEGs have much better file sizes than PNGs. PNGs should be reserved for images that absolutely require transparencies.
Always use "save for web." If you're using Photoshop, try to use the "save for web" feature - this will automatically apply some defaults that will be web-friendly.
Always use sRGB. Color profiles are tricky business! For the web, always use the sRGB colorspace. While CMYK is preferable for print processes, CMYK is not web-friendly as not all browsers are capable of respecting this color profile. In addition Vulcan now converts CMYK image uploads to sRGB. This can make the image look different in Vulcan than it did prior to upload.
Always upload the largest pixel dimension you have. Note that pixel dimensions are independent of "DPI" or "PPI" in your image editor. To make media assets flexible and high-quality, upload the largest pixel dimensions you have available (e.g. use 1200x1200 rather than 800x800)
Gifs - we recommend not using GIFs as Site Image or in your stories as they tank the performance of the page. We recommend using clips instead of GIFs.
Clips, a separate content type from Photos, can be a maximum of 30 mb and must be less than 20 seconds long.
Copilot will optimize images once they are uploaded. Images do not need to be formatted to any specific size. Anything under 30mb will be acceptable.