There are many ways to add a border to a picture in Photoshop. The following is a simple and quick method:
Click on the image you want to put the border on and then go to Select > Select All. This will put a selection box around your entire picture.
Go back to the select menu - Select > Modify > Border and set the size of the Border to suit the picture.
Their will now be 2 select boxes around your photo.
The final step is to fill the border between the pair of selection lines. You do this by: Edit > Fill. You have a selection of different colours and blending modes. Experiment to choose something that suits your picture.