Open your Google Docs document
Click the image you want to flip
Click the three-dot menu in the image toolbar, if available
Select Image options
Go to Size & Rotation
Use the Flip horizontally option to mirror the image
Use the Flip vertically option to turn the image upside down
If the flip option is not available, copy the image into Google Drawings
In Google Drawings, click the image
Open the Arrange menu
Select Rotate
Choose Flip horizontally or Flip vertically
Click Save and close to return the flipped image to Google Docs