Adding images/files to the "offline.json" "FileList" seems to work as desired. If anyone reads this in the future I have included some additional instructions below.
When you open up "offline.json" add your extra files to the end of the "FileList:" separated by a comma. The example below shows (3) images in the file list. For demonstration purposes all the other files C3 includes I did not show. You will need to update this offline.json every time you export a new version. You would simply add "image_1.jpg", "image_2.jpg", "image_3.jpg" to the end every time you export your project.