Right now I think you can only flip (mirror) an image when you
1) upload a starting image in the Meme Generator,
2) upload another image to add to a creation already in progress, or
3) are adding an image to a creation in progress from a template.
When using the Meme Generator to start a creation from an existing template, only the older rotate button is available.
Is your idea for Imgflip to add the flip (mirror) button to the Meme Generator so you can use it there from the start?