In PS my fave way to stroke is to use the Layer Style>Stroke. It's more editable, and you can fill the stroke with a gradient too. Inside makes sharp corners, but if you stroke a selection on a new layer, you have to enlarge it to show around the photo.
In PSE, which doesn't have the Stroke Layer style (though I cheated and made my faves in PS and saved as an asl file I use in PSE...) I also frequently grab a small square brush and fill in the corners. 4 quick clicks.
Another fast way is to grab a square brush, click on the image, and hold shift then click again on the other corner. If you're not too careful it gives a cool, not-so-perfect paper cut out look

have fun! Lots of options

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Emma the Pal
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