Thanks Edd, Just seeing this now.
I did not do anything as yet as I was working on other parts of the project. So far, just using a single image jpeg about 400-600 px wide seems to work OK on Android and iPhone plus iPad mini. I have yet to try a full size iPad, or Android tablet.
I will play around with PNGs thanks, as I’d like to get the filesize down.
I came across this topic which may be of interest to you: