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.
Do you want the returned image to be random?
If it's supposed to be the same every time, I think you have to pass `random_state=12345` here as well, no?
(and maybe the name should be changed to `make_100_gaussians` instead so that people don't think it's random).