Photo quality change after exporting

Hello

I'm working on a short film project that should be made from photos not videos (so photos are critical in this iMovie project), but after exporting the movie ( in HD) I found out that the photos' quality were reduced?

Most of the photos are darky, with dark backgrounds, they look good enough in iMovie but after exporting they look lighted up, which ruins my movie.

What should I do??

Thanks in advance