Add background pictures on jobs

Hi @pellepeister,

This small new feature enables jobs to be decorated with a picture in the background. It adds a position:absolute "<div/>" with an opacity:0.6 background-image so it does not interfere with the text.

You can specify for each job the url of the picture you would like to put on.

The "no picture" jobs are still displayed without bug.

Regards,
@libetl. (Lionel)