If the problem only appeared just after changing an image, maybe make sure the filename has remained the same. Double check the files' structure just in case.
Also, the zip should not be a folder with all the game content in itself. Just zip all the files and folders directly without a folder having all of them.
Cheers,
Aitor
Edit: Make sure you're also using the latest 1.2 version of the game. I worked on 1.0 version and solved my black screen problem updating to 1.2 version.