Hi, Welcome to JustAnswer. This is Mahbub Shahin can assist you.
At first plug in your usb flash drive into your computer.
Now open your image folder which one you want to back up
Press and hold Ctrl + A to select all images-->right click one of the image->send to-->removable drive.
After that your images will copy to your usb flash drive.
Any more question please let me know.
If you are satisfied please do not forget to click on accept and leave positive feedback. Bonus will be appreciable. Thank you.