I would recommend to upgrade your project to 5.18 or higher. So you can use the new page templates Optimized for Tablet. Or upgrade to the Mendix 6 version to get the newest features. Check link for more info: https://www.mendix.com/blog/the-eye-catching-mendix-ui-framework/
Image widths can be made responsive to the screen width, see this page for more about this. It is advisable to check each page/image for the need to make it scale along with the screen width.
I have created a design for desktop/tablet/phone but in the end I only use the phone design. So I can recommend you to use the phone layouts & designs for tablet too. If a page has more options then use a different page but otherwise use always the phone layout because users will find it more comfortable then a different design on each device. try to use % as much as possible instead of pixels!