Use Windows Explorer, find the folder you keep your images in (possibly called "pictures"), select it, right click on it and select "Properties" then look at the size it says it is.
Then choose the USB drive that is larger than that, as above - better still get 2 as they are not that expensive so you can make duplicates.
Copy the images to the USB drive.
Remove the drive.
Shut down
start the computer and plug the USB drive into another USB port.
Copy the images all to a new folder on your hard drive - this is to verify that they are correctly saved.
Label the USB drive - this is important - labels will remind you what it is when yo have forgotten.
Put them both in different safe places.