Without actually seeing what you need, I believe that I have a suggestion. First, create a new layer. Using the selection tool, draw a box, oval, or whatever size frame, over the image. Start far enough inside the image to account for your frame size. Go to the selection menu and invert your selection. Also, there are several additional options in the selection menu which will allow you to create a border, expand, contract, etc... However, be aware that inverting the selection will provide a "cleaner" frame than using the "border" option on the selection menu, unless that is the effect you are looking for. At this point you can fill the new layer with white. Go to the "blending options" menu by right clicking the preview window on your layer. Now you can add an inner shadow which will give the impression of a drop shadow. Actually using the drop shadow will cast it on the pic rather than in the white.
I wanted to add another suggestion for framing technique that may help you now or in the future. With the original inverted selection, prior to filling with color, use the quick mask feature at the bottom of your tool menu. Once the image is masked, should be everything INSIDE the frame, you can play with different effects to give your frame a unique look. There are literally hundreds of different combination's that you can try.