The Lenna image used in image processing tests is considered problematic due to
its unclear copyright status. Right now it is considered to be "overlooked" by
the copyright holder Playboy.
A suitable replacement image is already provided in the libav sources which is
licensed under the Expat (MIT) license and thus truly free.
This replaces the Lenna image with the one from the libav project.
This prevents extra IO. Different defaults are used for async backends
since we can’t assume that `existence_required` resulted in existence
synchronously.