Back to size.extension url format, because with a new parameter (background color),  and been two optional, was tricky.
New parameter in url: background color. Starting with tests.
Adding monolog.
New url creates an image (but without correct parameters).
Testing background color in image create with the new url.
Personalized background color now works. Refactoring tests.
Refactor.
Testing unknown background images.
Refactoring unknown background color uris.