Hanna,
This is typically something done at the CSS level and or JavaScript level by a front end developer.
Secondly, if your side isn't currently setup to be responsive meaning your content doesn't change "shape" for different viewports, this will need to happen as well. Again, something typically a front end developer usually does.
Typically setting the img width property to 100% will adjust the image but may cause a lot of problems I'd the rest isn't implemented. If you need assistance, look me up, we can help out.