Which OS are you on?
Have you assigned enough RAM under Preferences >> Performance? Does the error happen for all images types? Try Save As and see if the issue persists.
I had the same issue.
Maybe your issue is the same.
From within Finder, find the file you are trying to edit and right click to select "Get Info"
When the Get Info box opens scroll to the bottom and see if the file is locked.
Should be a small icon of a padlock.
If the file is indeed locked you may need to input your system password to unlock it.
If the lock looks closed then the file is locked.
Click the lock to toggle between locked/unlocked.