Bug1035492-[Wallpaper]-Use Img element instead of background images for gecko to handle decoded image memory r=djf

I don't know what, if anything we do with alt text in FirefoxOS. Would the wallpaper filename be a helpful thing for users who use screen readers?  Though presumably those users would not really care about changing their wallpaper...  I don't need you to change anything here. But I don't actually know why you're setting this attribute at all.