Try experimenting with images of different aspect ratios, to see if you can find a ‘sweet spot’ where you’re happy with the sizing on each of your devices.
Ex. if you’re using an image that’s 1300x600, then try maybe 1200x400 or 1600x500.
Play around with the image sizes until you find a size that works best for how you want things to look. With that said, it can sometimes be hard to find an image size that works perfectly for all device types.
Most premium themes do have an option to upload a secondary ‘mobile’ image though (it’s usually right below where you upload the image for desktop). Having 2 versions of the images (landscape for desktops, and portrait for mobile) can help with this. If your theme doesn’t have this functionality built in, you can always try switching themes to one that does. You can test themes before purchasing by adding the demo to your store, and then exploring the settings built into it via the theme editor.