Need test coverage

[jakelowen]

After we implement solution to #5 (and implement other checks), we need to cover these with tests. api.spec.js has a good template example to pass in a POST request payload and expect a certain result

[jakelowen]

It would be very cool to even include "visual regression testing" i.e. https://github.com/americanexpress/jest-image-snapshot to make sure images are generated as expected.

Alternatively, I wonder if base 64 images are idempotent - or in other words perhaps we just test for exact string equivalence of base64 image string? Might be a good use case for normal (non-visual) snapshot testing.