I looked on iPhone and the sizing of the whole website seems off. Looking at the source code (might not be 100% cos of the app I’m using) your viewport metatag looks like this;
<meta name="modulate" content="width=device-width, initial-scale=1">
If this is what’s actually in the source and not what the app is pushing through, you ought to correct the “modulate” part because the viewport metatag should look like this;
<meta name="viewport" content="width=device-width, initial-scale=1">
If you make this correction, your website should adopt the expected sizing and responsive behaviours.
This won’t necessarily fix your image issue but you ought to start with the correct foundations for a mobile site before editing for mobile.