The tests only use standard method calls. So the "real use" and "test" dependencies should be the same.

This is weird. On my machine: graphicsmagick __not installed__ + imagemagick __installed__ = tests passing. Can you try uninstalling graphicsmagick and run tests?

The other part that I'm confused by is that pHash does not list `libpng` as a dependency ([see downloads page](http://www.phash.org/download/)). Based on tests failing without imagemagick, I think it should be listed in README as a pre-requiste/dependency.