WordPress introduced responsive images back in 4.4, meaning that it will make its own scaled-down versions of the images and your theme will serve the closest (but still larger) scaled version so your viewers aren’t stuck loading huge originals all the time: https://www.ads-software.com/news/2015/12/clifford/

What about retina screens though? How does one strike a balance between image dimensions and file size when such computers are used these days?

WordPress makes so many scaled versions of the original now to accommodate a wide variety of screen sizes. Just upload the largest size you’re comfortable with, and WordPress will always serve the appropriate pre-scaled version to each visitor depending on what the theme needs and the screen size of their device (including whether or not it’s a Retina device).

I realise this is getting technical so please bear with me. Does the fact that WordPress generates many scaled versions mean that one is best advised to upload a higher quality — meaning larger file size — image to prevent image degradation as WordPress upscales or downscales the image? Or will that slow down the drawing of a page so that one had better upload a lighter image?

br
Philip

Does the fact that WordPress generates many scaled versions mean that one is best advised to upload a higher quality — meaning larger file size — image to prevent image degradation as WordPress upscales or downscales the image?

Yes, exactly that. ??

Or will that slow down the drawing of a page so that one had better upload a lighter image?

The point is, WordPress will make lots of smaller-sized versions of the uploaded image and always display the best possible one based on what the theme needs and the screen size of the viewer, never the full original (unless that’s what the screen size called for, of course).