It's easier this way

It feels unnecessary. Ultimately a background is as simple as a path to a file or URL... so there's no real need to create a whole new class just for a new background

The underlying encode() method will only accept an instance of an EncoderInterface, so passing a string here just doesn't make sense

If the user wants a different format to PNG, they should pass an instance of the relevant encoder

# Conflicts:
#	tests/test.php
#	tests/test4.png
#	tests/test5.png