Seems that the issue was a lock file with conflicting permissions. Most likely the system created the lock but then it wasn't able to delete it due to wrong permissions in that folder.
I've deleted the contents of app/content/locks and the auto-delete worked fine. I also checked that the system deleted all the previous expired images.
If you have issues in the future, simply wipe the contents of that folder, or better, provide full permissions to PHP to be able to handle the contents of that folder.
Cheers,
Rodolfo.