I have done this by using the Hide on check box at the bottom of the properties display (when selected on an image). I "hid" the larger image on the smaller formats and added a different image, which I then "hid" on the larger formats. Seemed to work out OK in my application.
That's interesting advice ptrader.
When you checked your page on pagespeed insights, did it penalise you at all for using this method?
Last edited 21 February 2019 by marcus60