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what outside device and how did you "install them"
usually you would copy them to your pictures folder but if another program imported them than they might be in its own folder or even in the cloud somewhere
you can try searching the computer from the libraries folder.
in the top right corner is a search box.
put *.jpg or *.png and it should find most of the photos on your computer
its also possible that rather than copying them to the computer you were just looking at them while they were still on the device and they would have to be copied again.
I can get you my phone number and we can talk and i can log into your computer remotely to look for them with you. Or if you still have the device we can just import them again and i can make sure that you know where they are