We forgot to do so in commit f0ab7bcfc. We're still leaving one system
spec to test that these images are rendered when browsing the site.
We're also changing the existing component tests to check the `alt`
attribute, just like the remaining system test does.
We forgot to change the line rendering the image in commits 3574bf867c
and 810bdae37a, and so the custom image was being ignored.
Note that, in the test, we're stubbing a constant instead of adding a
new image. The main reason is that, if we add a new image, forks would
have to change the image when they change the `VALID_IMAGES` constant;
otherwise the tests would fail.