Dallas from the Shopify Social Care Team here to help.
What you're currently dealing with is a difference in the mobile's aspect ratio so I am more than happy to clarify for you. Images on the desktop are set to be viewed horizontally since that is how the screens are designed. On mobile however, the screens are vertical so the websites are set to be viewed vertically. The image below is a great example:
When a site is viewed on a mobile, the entire websites moves from the horizontal layout used on the desktop and as a result the images are cropped to fit the aspect ratio of the mobile. The webpages are adjusted in regards to layout and the photos are cropped so the aspect ratio of the original [horizontal] photo will fit the aspect ratio for the mobile [vertical] phone. Just like the edited image I attached above.
If all the pixels that make up the image were to show on the vertical mobile layout all the pixels in the area that have been crossed out with pink will have to fit into the pink rectangle box. That would make your image look squished and unnatural.
Since you have paid for your theme it isn't a Shopify created theme so I can't get our themes team to do this for you, but something you might be interested in doing is speaking with the theme developers to see if this is something that they can help you to adjust. You'd want to ask them if there is a way to allow different images on the home page for mobile than the home page for desktop. If they can't help you, you might be interested in speaking with someone in the Expert Marketplace.
That should help to clarify, but if you have any more questions please let me know.
Can you please give me your store url so that I will give you solution
If you know the name of the theme then you can go into the Shopify theme store and type the name. On the theme page in the theme store there is a section where it gives all of the information for the theme desinger.
For example this is what it would look like:
If you're not sure the name of the theme you can go to your Store Admin and go to Online Store > Themes > Customize. Under Themes Settings at the bottom of the page on the left, it will give the name of the theme and the developer. You can look them up on Google if you would prefer to just do that.
If you let me know the name of the theme I can even get the contact information for you. Please let me know if you need more help.