Background image brightness control

I would like to be able to control how bright/light or dark a background image is. I don't always want to cover up a great background image with lines so that the lyrics can be read. Is there any way to add some kind of sliding brightness control for images? Thanks for all you guys do!

Just wondering the same thing. Does anyone know how to change the background brightness? The sunlight affects the ability to read the words, unless it is dark enough. Thank you for all your help! :)