Thank you Richard.
Yes, I am using iCloud photos
I have turned on Optimise storage
No, I cannot edit the picture at all.
Yes, checked the settings as you suggested.
The jpg is a saved pdf from a WORD document which was then imported to Photos.
The original image was a jpg onto which I have placed text when in the WORD format.
However, also, I imported a new photo jpeg into Photos, and it did the same blurry response when in the edit-crop mode. And the edit function was inactive. I restarted the iMac and it did it again. Then I repeated and it cropped. But the original jpg, which is the basis of this enquiry, remains unworkable.
Any thoughts welcome!!