You may have added images to a project and later deleted that image from the file library. After some time you may reopen that project and the image will be gone, as the system will remove garbage and deleted images permantly from the user's account.

To fix this, add the image again to the file library and add it to your project. On the right side of the studio in the layers section, you will see an "Empty" image. Click on it to bring up menu and delete that "Empty" image.