Very commonly asked question. Here's a code pen showing the issue.
Computer Monitors are completely different dimensions than phones. You can see in the first box, that if your picture scaled to your phone screen width it would become quite small. What you're probably seeing is the third box. This is how themes handle background images, they have the background image set to cover the element, the element has a set height, and the image is centered. So it has to be cut off. I would redesign it to use a mobile image and desktop image and swap them out with CSS based on their screen widths. This all requires quite a bit of custom code knowledge though, so if you don't have that your best option would be to hire a developer to create this functionality for you, or make sure you're picking an image where the focus of the content is mainly in the center of the image.
This is an accepted solution.
It's possible, but think about your overlay text. How is it going to fit on such a small picture? Or if you have text in the image itself, the text will also shrink to a very small size. It would probably have to be redesigned as I'm sure your theme is using a background image instead of letting the image element itself define the container.