Copy the override code and then, go to your Assets tab, click on + and add a new override. name it "FlipImage" and paste the code inside replacing the reference code from Framer.

Select your frame/image and from the right panel go down to the code overrides section and click on + then select Flipimage on file and either FlipHorizontally or FlipVertically on the override.

To note that the effect is not visible inside the canvas, only in preview, this is a limitation on how overrides function inside Framer.