Adding a background image with a gradient overlay to a flip box is not a native capability in Avada, but it is easy enough to do with custom CSS. Make sure your flip box does not have a background image for the front.
- Create a column and add a class (I used eagle)
- Add the flip box and edit the content and titles
- Create a custom CSS entry (I recommend at the page level unless these will be repeated often).
As you can see in the code sample, I added height and padding to my flip boxes so that they are uniform. I did this because all the flip boxes are in a separate column which gives you a great deal of control.