Sometimes people give me photos that are locked. I suppose they were locked while in their camera in order to protect them. But that means, I can't change or delete the files. It is quite simple to unlock a mac file. Just locate the file in the finder, highlight but don't open, and then press the command+I (for info) keys. This brings up an information window. About halfway down the window you will see the word 'locked'. Click on checked box and it will uncheck and unlock.
Sometimes I end up with locked files in the trash and it won't empty completely. Just double click the Trash, select the files and follow the steps above to unlock.