Another option for this particular type of problem is to let people upload files in whatever format and size they want, and then use an image resizing proxy (such as imgproxy, or one built into your app using the Image library) to resize them at runtime, and then place that proxy behind a CDN.

That way, you can change the image size in HTML and when you deploy, all of the images will magically be the new size. It also allows you to have different sized images for phone vs desktop layouts, and lets you easily supply multiple resolutions of an image and let the browser decide which is the appropriate one to pick via the srcset attribute.