This is something that I've learned recently, and the two here are my 4th and 5th attempts... Below are the instructions...
Directions (Photoshop Elements Version):
Choose a photo: Once you have selected your image and opened it in Elements, duplicate the image (Image>Duplicate), close the original without saving it, and save the copy in your projects folder.
1. Duplicate the background layer and make the Background copy active. Click the new layer icon at the bottom of the Layers Palette to add a new layer above the background copy layer. Double-click the layer icon for the new layer, and name this layer Frame. You should now have three layers: Background, Background Copy, and Frame (the top layer). Make the Frame layer active. Get the rectangular marquee tool, and select a portion of the photo that is to be (roughly) the in-bounds portion of the image. Set the foreground and background colors to the default black and white.
2.Making the white frame: With the Frame layer active, go to Edit>Stroke. Use Background color (white), set the width to about 15 pixels (this depends on the size of the source photo), and the position to Inside. Click OK. If the frame is too wide or narrow, undo and stroke again with a different setting. Deselect when you are satisfied.
3. With the Frame layer still active, go to Image>Transform>Perspective. Adjust the frame to give the perspective of a viewer looking across the in-bounds portion of the photo. This adjustment depends on how you select the point of view and how you want the elements of the photo to be positioned in or out of the frame. You may want to use the Distort Transformation as well. Do all of your transforming before you commit the transformation.
Setting up the clipping group structure;
4. All of the photo outside the frame except for the portion you wish to keep will be removed. These remaining portions of the photo that are to be on or outside the frame need to be extracted from the rest of the photo background--some parts will appear to be on top of the frame.
5. To extract those OOB portions of the photo, we will use a fake masking technique so we don't alter pixels in the photo and can refine the extraction as much as we like. In full-version Photoshop, you would use a layer mask to do this, but Elements does not have layer masks. We will simulate a layer mask by grouping the photo to a base layer and “painting in” the portions of the photo we want to reveal. To edit the photo, you simply continue to paint pixels on the base or erase them from the base. Here's how you need to set the layers up:
6. At this point you should have the following layers: Background, Background copy, Frame. Make the Background layer active and put a new layer above it. Name this layer Solid Color. This will be the solid color background for the OOB image. Select a light color for the foreground color and fill this layer with it (Edit>Fill, use Foreground color). With this layer active, click the new layer icon to add a new layer above it. Name this new layer Photo Base. Double-click on the Background copy layer, and rename it Photo.
7. Now you are going to use a clipping group to isolate the framed in-bounds portion of the photo. While you are doing this, you will set up the layer for the shadow under the photo. Set the foreground and background colors to the default colors of black and white. Make the Frame layer active. Get the magic wand tool and use the following settings: Tolerance: 0; Contiguous, checked; Use all layers, unchecked. Click outside the frame to select the region outside the frame. Now do Selection>Inverse to select the frame and the portion of the photo inside the frame. Make the Photo Base layer active. Go to Edit>Fill, and fill the selection with the foreground color (black). You should have a black polygon on the Photo Base layer in the layers palette, but you won’t see a change in the image, because the polygon is under the photo. Deselect. Drag the Photo Base layer to the new layer icon to duplicate it. You should now have two layers with the black polygon—name the lower one Photo Shadow. The upper one should be the Frame Base layer. Make the Photo layer active. Go to Layer>Group with Previous to clip the Photo layer to its base. Everything outside the frame will disappear, and you will see the framed photo on a light green (or whatever color you selected) background. Note: instead of using a clipping group, you could have simply deleted the portion of the photo outside the frame. But you need that black polygon anyway for the shadow….and it’s good practice for the next step.
8. Now you are going to set up another set of layers and a clipping group for the out-of-bounds parts. Drag the original background photo to the new layer icon to duplicate it, and move this copy to the top of the stack above the Frame layer. All you will see in the image now is the original photo with no frame (because that’s what’s on top of the stack). Rename this copy of the original photo OOB. Make the Frame layer active, and click the new layer icon to add a layer above it. Name this layer OOB Base. Make the OOB layer active, and do Ctrl-G (Layer>Group with Previous) to clip the OOB layer to its base. Since the base layer in this clipping group is transparent, the original photo on the top of the stacking order disappears (is completely masked), and the image now looks like the framed photo on the solid background again.
9. Now you are going to reveal the out-of-bounds parts of the photo. Make the OOB Base layer active. Be sure the foreground and background colors are still set to the default black and white. Get the brush tool and select a soft brush. Paint on the OOB Base layer in the general areas that you want to reveal, and you will see the photo beginning to emerge. Use the erase tool (on the OOB Base layer to remove the portions you want to exclude.
10. When you are satisfied with the extractions, make the Photo Shadow layer active, and change the blending mode on the layer to Multiply. This will be the shadow for the photo itself, and you are going to blur it (Filter>Gaussian Blur). But you may want to turn off the layers above it so you can judge how much to blur it. When you are satisfied with the setting, click OK. Turn the upper layers back on, and position the shadow so it is consistent with the shadows in the photo. Scale the shadow or reduce the opacity of the layer until you are satisfied.
11. Make the Frame layer active. Click the new layer icon to put a new layer above the Frame layer. Rename this new layer OOB Shadow, and change its mode to Multiply. Get a soft brush, and paint shadows for the areas of the photo that are on and outside the frame. Be guided by the placement of the shadows in the original photo—erase and repaint as necessary. When you are happy with the general shape and placement of the shadows, blur the layer (Filter>Gaussian Blur), and reduce the opacity of the layer.
12. Save as a .psd before you flatten the image so that you can go back and make changes later.
For this one, I used a monarch catepillar photograph and exctracted the background, then added it as the top layer, adjusted the perspective and size and positioned it over the top of the frame.