Open the Google Docs document
Right-click the image
Select Save to Keep
Open the Keep note in the right sidebar
Right-click the image in Keep
Select Save image as
Choose a location and save the file
If Save to Keep is unavailable, copy the image
Paste the image into Google Drawings or another document
Download or save the image from there
On mobile, tap the image
Tap Copy
Paste it into an app that allows image saving
Save the image from that app