Open your Google Docs document
Click the image you want to flip
Click the three-dot menu in the image toolbar, if available
Select Edit image
In the image options panel, open Size & Rotation
Use the Flip horizontally option to mirror the image left to right
Use the Flip vertically option to turn the image upside down
If Flip options are not available, insert the image into Google Drawings and flip it there
Save and return to the document