Its called "stroke" and this is how you do it...
ctrl-click the photo you want to have a white border. Then go to -edit- and then -stroke- and choose your color, placement (I do inside), and width (I do between 15-35 depending).
And voila! There ya go!! instant white border!!
ETA: stroke is also a good tool for bordering an alpha or some other element to make it look like a "sticker".
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Volcano digital review