Hi,
There are several ways to flip images.
The two that come to mind immediately is that of rotating the image 180 degrees and the second is to rotate the camera 180 degrees.
Both methods can be done through the Function Editor.
If you open your Funtcion Editor you'll see in an adjacent window a listing (I like to refer to the listing as the Project Workspace but that is a term used in other software).
In that listing we'll find all the objects and effects contained in our project.
Cracking open that list reveals finer detail on what properties can be changed; translation (movement right, left, up, down), rotation, scaling (horizontal and vertical), etc.
These settings can be keyed (for use with animation) but they can also just be set once and persist throughout our sequence.
So, find the object of your choice.
Open the listing to where you can select the property of your choice
See that setting in the Function Editor (looks like the xsheet)
Enter the desired value on the desired frame.
My apologies for the small size of the gif demo.
It's just meant to demonstrate that it can be done.
If that doesn't get you there let me know and we'll reattack.