I wouldn't use a motion path at all. I'd add a 3D null to the scene, arrange the photos around the null, then animate the null's rotation and position. Now the photo's transform properties become relative to the null, not absolute in the world, and you are able to easily layer animations to build complexity.
You can also do this with a light and use the light as an emitter with Particular to get some pretty complicated spirals very quickly without fussing with 3D motion paths.
Walter Soyka
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