Background fill not working.

Check the attached example, I uploaded a red box and it filled the background with black.

![fill](https://f.cloud.github.com/assets/1651502/1368219/ca49b8ca-398b-11e3-8580-d1fbbd132050.png)

Example code to see this in evidence:
`echo '<img src="' . wpthumb( 'http://s8.postimg.org/l0pouom41/image.jpg', 'width=500&height=500&crop=1&background_fill=solid' ) .'"" />`