Hey there @PaulineLomax ,
You can increase the size of your images on mobile using some custom code. Are you able to provide your store URL?
Hi @PaulineLomax ,
Please share your store url and password if it is protected.
I will able to guide you once i will see the store design.
Do you want to show images in the full width in the mobile like below image, right?
If yes then please DM me then i will help you to customize with your theme.