How do I make a black line around an image on a white background for a rubber stamp?
If your image is the size you want to use for the rubber stamp and there is sufficient room around the artwork.
Try selecting the entire image.
Then contract the selection by the number of pixels you want the border to be. In my experience, approx. 8px will make a border of approx 1pt, and using multiples of 8px for thicker borders.
Next you invert the selection. Pick a color (i.e., black) and fill the selection. When you deselect the mask, you should have a border around your artwork.
You can also try the frame feature in photoshop.
*One thing to keep in mind is that you don't want too thin a border for a rubber stamp. Usually lines that are thinner than 1pt will give you wiggly lines once the stamp is produced.