When i try to export a project, some parts of the file turn black

I have a school project that i made with openshot. I have a .ogg file for the sound, and i have .png images throughout the project. I have no transitions, only static images wich change. Pretty simple project, but it lasts for 20 minutes. Whenever i press export, it always has large chunks wich are kinda blacked out. The project deadline is tommorow, so any help would be greatly appreciated! Thanks!