Preview Not Working

For some reason this particular project is giving me problems with Preview. I tested another (much larger) Storyline course I made that works great. I have also had a co-worker test the Preview of this course and he has had similar issues.

Is it because of my background images? These are images that I have scaled to have a canvas size of 720x540 pixels to make them easy to work with and not too large. That is the only difference I can think of between this course and others I have made, although truthfully in other courses I work with very large images frequently with no problem.