Welcome to Android Central! If you're using the Google app for image searching (and not the Google website in the Chrome browser app), then you have to make sure you turn on Storage permission for the Google app first. Go to Settings>Apps, select Google, then Permissions, then turn on Storage (it's turned off by default, for some reason). Then any image you download from the Google app will go to the /Download folder. When the Storage permission is off, it gets saved to a protected system directory that can only be accessed by the Google app itself.