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

I'm not sure this comment helps at all. A future reader won't know what has actually changed here.  You're not proposing that we ever change back to using backgroundImage, so it is not like we'll have to undo this change later.  So I think that referencing a bug number will just be confusing. If you think it is useful you could comment that we are purposely using an image tag instead of backgroundImage because gecko handles memory for off-screen images better in that case.That could be a useful warning to future maintainers not to switch back to backgroundImage...