1. Open your picture file and crop it to the desired dimensions, or, go to Image>resize>image size and resize it with Resampling & Constrain Proportions checked. Note that for printing it is desirable to have the resolution in the 240-300px/in range.
2. Go to Select>all (CTRL+A) to select the the entire picture
3. Open a blank layer at the top of the stack of layers and go to Edit>stroke(outline) selection. In the dialog place the stroke inside, select the width of the stroke, and the color of the stroke. (white for your project)
4. Press CTRL+D to get rid of the "marching ants" outline. If you are not happy with the stroke, delete the stroke layer and repeat the drill to suit.