Picture attachment fails-why?

This is on an iMac, Safari, various pictures attach fine but a couple fail right at the end. And, if I try to attach another copy of the file from another location (backup drive for example), it still fails.

They open fine on the computer.

???

 

Mmm, just noticed the picture said "locked" in Preview, but in the properties it is not. When I tried to duplicate, it said the picture was locked because I had not mumble mumble something in a long time, and I could "unlock" and then attachment was OK. But what the heck is the locked/unlocked bit?