Open the photo you want to download
Right-click the photo
Select Save image as
Choose a folder on your Chromebook
Click Save
If the photo is in an app or website without right-click, look for a Download button
If the photo is in Google Photos, open it and select the three-dot menu, then choose Download
If the photo is in an email, open the attachment and select Download
If the photo is on a phone or camera, connect it with a USB cable and copy the file to your Chromebook