I am on a MacBook Air and using a guest account. I am trying to access a USB, but I can't
The reason this is happening is because &quot;File Vault&quot; is activated on your MacBook. File Vault is a program that comes with your Mac and encrypts your hard drive so only people with the password to do so can look at files on your Mac.
Apparently once File Vault is activated, you can only access Safari on a guest account, as stated here.

If you log in as that guest user, you don't get access to your hard
drive; you can only access the Safari web browser and a network
connection. Exiting Safari simply returns you to the FileVault 2
pre-boot login screen.

This means, you cannot save e.g. images to your computer and then put them on a external drive. This is (as far as I know) because files are not uploaded directly from an USB device to the Internet: your computer performs the task of moving the file from the USB device to the hard drive and from there to the Internet.
I'm afraid as long as you can't change the settings with the Admin Account (in this case you'd have to deactivate File Vault) you cannot save files to your computer or an external drive.
What you could do instead: access your e-mail client within Safari (if you have Gmail, just go to Gmail and then right-click on the picture you want to get and choose &quot;Copy Image URL&quot; and then paste the URL in your e-mail and have it send to yourself (just send an e-mail to your own e-mail address) With this way, you can download it on another computer, once you access the URL of the picture.
Google's instructions for this:

Computer
Do a search on images.google.com for the image you want to find.
In your browser’s address bar at the top of the page, click once
to select the entire URL.
(If you're looking for the URL of an image
result, you need to click on the image to open up a larger version
before selecting the URL.)
Right-click the image.
Depending on what system you’re using, select the image URL by clicking one of these:
Chrome: Copy image URL
Safari: Copy image address
Internet Explorer: Properties &gt; Copy the URL address shown
Firefox: Copy image location

I hope this wasn't &quot;too easy&quot; for you and let me know if it works out!