Open the Google Slides presentation
Select the slide that contains the image
Right-click the image
Click Copy
Open a new Google Doc, Google Drawing, or image editor
Paste the image
Right-click the pasted image
Choose Save image as or Download, if available
If Save image as is not available, use File > Download and select an image format
Alternatively, take a screenshot of the image
Crop the screenshot to the image only
Save the cropped image to your device