If you have different content layers transitioning in and out at different points in your banner animation, you will likely want to hide some of those from appearing in the your backup.jpg file.

If you would like to hide any layer in your timeline from being included in the backup.jpg file with your HTML exports, you can optionally do this for any layer by clicking any layer's Settings icon button in the main timeline to open the layer settings, and enabling the Hide this layer from being included in your backup.jpg toggle.

This will automatically hide that layer from appearing in your backup.jpg file during the export of your HTML banners.