You can easily flip images, using the flip function found in the layer settings.
Image Flip in the Editor
Using the flip function in the editor can be done from the layer section of the toolbar that comes up when selecting the specific layer that you wish to modify.
Image Flip in the Generator
For the Banner Generator, you can apply one of two available methods, depending on your current preferences.
The first method applies the function to a specific layer on all sizes, whereas doing this in Focus Mode(single edit mode) will execute the process only on the size which you are currently modifying, similar to the process that takes place in the Editor.