Specifying Custom Dimensions For A Shield Ui Background Image

I have a site where I am providing users some graphical information and I use the Shield UI JavaScript Chart.I am trying to take some more advanced use of the chartAreaBackgroundImage: property. I am actually in need of several things I couldn’t find anything in the manuals.

Is there a way to resize the picture by a certain factor e.g. to specify exact dimensions?

Is it possible to set transparency for the image used?