The guy that Francois linked obviously didn't follow Charles' advice to get rid of dust, etc. in PS first.
I used a few instructions to try to do this and there were slight variations out there, and what I came up with was basically Charles' with the exception of not using Auto-align when you create the stack of four images but instead aligning manually. And I'm a PS noob so I need the very detailed kind of instruction. So, it goes something like this:
1) scan all four images in a single scan to assure consistent image characteristics across all four images.
2) adjust levels, curves, etc. to your liking on the single four-image file.
3) clean up dust per your liking.
4) save the file.
5) create four separate image files (by cropping and Save as... under a common name with sequential numbers 1, 2, 3, 4 at the end of the file name).
6) go to Files > Scripts > Load files into Stack... Browse and select the sequentially numbered single-image files; do not check the "Attempt to Automatically Align Source Images" box.
7) you now have a stack of four images with each its own layer; working from the bottom up, click off layers 1 and 2, and turn layer 3 opacity to about 30% (keeping layer 4 opacity at 100%); using the Move Tool, align the ghost image of layer 3 to the underlying layer 4 image using the subject's eyes or other feature you want to be the "central point" or axis around which the 3D animation swings.
8 ) do the same thing for the other layers: click on layer 2 and set it at 30% opacity and set layer 3 at 100% opacity; align the ghost image of layer 2 to the underlying layer 3 image; click on layer 1 and set it at 30% opacity and set layer 2 at 100% opacity; align the ghost image of layer 1 to the underlying layer 2 image.
9) make sure that all layers are clicked on and set at 100% opacity; you'll see that through the alignment process the frame edges are no longer aligned so you need to crop. Crop once with all layers clicked on, and crop again with only layer 4 clicked on (this way you get rid of mis-aligned edges on both the right and left sides).
10) go to Window > Animation and click on the drop-down icon at the upper right, and select Make Frames from Layers.
11) your four image layers should now appear as a sequence of four frames; adjust the time each frame will appear in the animation with the drop-down selector under each frame (0.2 seconds is a good start).
12) the gif you want to create will be a loop that runs continuously, so you want the frame sequence to be 1,2,3,4,3,2 -- so you need to copy frames 3 and 2 and add them to the end of the sequence, using the copy icon at the bottom of the animation window and dragging the copied frame in place accordingly; somewhere along here you want to save the four image stack and the animation you created (it's all saved as a single psd file).
13) to create the gif file, go to File > Save for Web and Devices... at the lower right, under Looping Options, select Forever; click Save and you have a gif file of your animation.
14) find a host, like tinypic.com, to host your gif file (upload the gif file as an "image"), from which you can link and embed and generally annoy friends and neighbors.
