When a Flexitive design is exported as a ZIP file, one of the files included in the ZIP package is a Backup Image.
A Backup images is a static JPG of your design, and most ad servers require a Backup Image to display your design. If something goes wrong when the design is being displayed and the ad server can't use the HTML5 version of your design, they will display the Backup Image instead.
In Flexitive exports, the Backup Image is created using the layout of your design. If your design is animated, Flexitive will use the last frame of your animation sequence to create the Backup Image.
If you would like to change the Backup Image in your design, you can duplicate your design and adjust the layout of the duplicate to better fit how you would like your Backup Image to be prepared. You can then export a static JPG of you design to use as a new Backup image.