Even later, but this could be usefull too. There is the jQuery-backstretch-plugin you can use as a polyfill for background-size: cover. I guess it must be possible (and fairly simple) to grab the css-background-url property with jQuery and feed it to the jQuery-backstretch plugin. Good practice would be to test for background-size-support with modernizr and use this plugin as a fallback.
The backstretch-plugin was mentioned on SO here.The jQuery-backstretch-plugin-site is here.
In similar fashion you could make a jQuery-plugin or script that makes background-size work in your situation (background-size: 100%) and in IE8-. So to answer your question: Yes there is a way but atm there is no plug-and-play solution (ie you have to do some coding yourself).
(disclaimer: I didn’t examine the backstretch-plugin thoroughly but it seems to do the same as background-size: cover)
