Open your image. On the layers box, right-click your image which name should be Background at the moent and click Duplicate layer. After it’s duplicated, delete the one that says ‘Background’.

Now you should only have 1 layer called ‘Background copy’. Now click the ‘add layer style’ button.

Select Stroke. Change position to ‘inside’. Here you have the option to choose a color, gradient or pattern.
Choose whichever you’d like and adjust the width. Done!