You can use the File Manager to browse the cards's contents and associate pictures and videos with programs that can view/play them

for pictures, you can use fbi as picture viewer (https://www.raspberrypi-spy.co.uk/2017/02/how-to-display-images-on-raspbian-command-line-with-fbi/) for videos, you can use omxplayer.

RetroPie already comes with usbmount already installed, so you just need to make sure it's enabled so that whenever you insert an USB stick it's automatically mounted by the system. You could probably customize the usbmount actions to always mount the USB device to a fixed location.