Screens (and images that play that role) are most of the time the main building blocks of user flows. One of the most common requirements is to show a device skin around those screens. This is done in Overflow through a switch in the Device settings. Another very important setting is Mask content to device. When the content is masked, then the screen becomes scrollable in Rapid Prototype.
Overflow detects the device based on the size of the imported screen (or inserted image size). If a known device is not detected, 'Custom' is selected in the device dropdown. In that case, you can also edit the mask area.