TST: Specify figsize in image test, overriding unknown default.

The most direct fix is to specify the figure size of the test figure. One can imagine related issues, and eventually we may want to standardize the rcParams in the test environment. But I believe there is ongoing work on mpl to handle rcParams a little differently, which will make it easier to create non-overwriting styles in the future. For now, maybe a simple approach is best.