First, get out your Freehand Selection Tool and set your settings as mine:
Now make any selection around your picture like mine:
Now copy the selection by clicking Ctrl-C and then make a new image 500×500 filled with black. Now click Ctrl-E and the selection should pop up.
Now go to Selections >> Modify >> Expand and choose how many pixels you want. Then click Ctrl-C then Ctrl-V and you’re done!