/series/1: add an image

[php-coder]

The pre-existing series doesn't have an image at this time. Let's add it.

[php-coder]

In integration tests for #199 I decided to use /image/1 as an image that can be downloaded. It turned out that it makes test fail because the image only exists on test environment and is absent on the travis environment (where we're persisting images to the file system).