With image renditions you can specify a pixel size when requesting an image, and SharePoint will deliver the image scaled down to this size for you. This is great, because you don’t have to worry about your users pulling full size pictures from their gigapixel cameras when publishing content. Or do you?
I was recently browsing a customer’s SharePoint Online intranet solution and noticed that some images in the news roll-up loaded slowly. Turned out they had been using high resolution stock photos for a few of their news articles. Even though the news roll-up requested a specific image rendition, SharePoint delivered the original, full size image file. The penalty of this is both downloading large files (10MB in this case) and having the browser scale down the image (which is slow and looks bad). I’m sure you can imagine the performance impact this could have on pages such as an intranet start page where you have lots of images.
I investigated this further by scaling an image to various sizes to see what happened. Turns out, at around 6400 pixels width or height, SharePoint suddenly stops delivering a scaled down image rendition and instead just sends the original file. This is true for both SharePoint Online and SharePoint 2016. My recommendation is that even if you use image rendition, user education is still necessary to avoid problems.