Thanks for your suggestion. This CSS workaround has effect. Great! The image distorts indeed a bit.
The actual model of the iPad is ‘iPad Pro 12,9” Retina display (2732 x 2048 px).
However the problem also occurs on an iPad min 3 (2048 x 1536 px) and on the old iPad WiFi (2012) (2048 x 1536 px).
I prefer not enlarge dpi because I also get a larger image file size which effects response.
The theme is updated to the latest version 4.4. Issue on iPad is still present (both Safari and Firefox).
Please have a look.