This happened to me once. It was a corrupt project.
Try opening a new project and importing the troublesome one into it.
Thanks for the response, but I just tried that and straight away I drag/dropped a picture and it was wrong
Maybe the image folder is corrupt.
Rename the folder containing the images. Create a new one with the name of the original, and copy the images into it.
In the bad old days of Windows 95-98 there used to be a fault known as cross-linked files where part of one file was saved with part of another!! Not good. Try a search for that online.