: This icon is courtesy of myself, makeuplovegloss, use if you'd like.
Open the image you want. Always be sure to copy the icon, and close the original, just so nothing happens to it.
If you already have the image open and you're working on it, make sure your image is merged first:
LAYERS > MERGE > MERGE ALL
Then select your entire icon:
SELECTIONS > SELECT ALL
Once you have that, you are going to want to make a screen cap of the icon by pressing the PRINT SCRN button on your keyboard, located to the right of the backspace key.
Once you have a screen cap, you want to paste it on as a new image:
EDIT > PASTE > PASTE AS NEW IMAGE
Once you have that, you need to crop the image just to the icon width, just outside of the dashed area. Make sure you have the CROP TOOL selected, and crop the icon area. Make sure your image is still 100x100, or whatever you started out with:
IMAGE > IMAGE INFORMATION
Then save the image, and you are all done. That's actually the easiest way to do it, so you don't have to physically go in with a brush and make hatch marks around the edging.