Add two new images to datasets

This adds two new images to datasets:  a small image with four 2D Gaussians plus Gaussian noise and a larger image with 100 randomly-generated 2D Gaussians plus Gaussian noise.  They are the example images generated in `make_gaussian_sources` and `make_random_gaussians`.

I plan to use these for docs and tests.
To make sure it's the same every time you could hard-code an assert on the image sum here?