How to get background image to fill the browser and remain fixed in both IE and Firefox?

Question

Basically what it says in the title. I've come very close in achieving this but something just doesn't tie up in the html and css code...
First of all, I'm using IE8 and the latest version of Firefox to test this.
I used 2 sources of information for getting this done: 1) http://css-tricks.com/perfect-full-page-background-image/ and 2) http://stackoverflow.com/questions/8958697/css3-background-size-cover-doesnt-make-image-co ver-vertically
In Case 1 the example on the page called CSS-Only Technique #1 gives code that works almost perfectly for me, except that obviously I need a background image, however the CSS provided there is clearly just for an image dropped in the body of a page with no regard for other content that may already be there. The html code that I put on the page is simply <img class="bg" src="../images/background_image.jpg">
Obviously I used the CSS code provided in the example, minus the last bit which is "@media screen" etc etc, which seems irrelevant to me. Obviously substituting values to suit the image on my page.
So what happened is that it almost worked as intended in both the browsers (which means the image filled both browsers width-wise as intended and also remained fixed when I zoomed in or out in each case) except that the image went over the top of the current content that I had there already precisely because it's not a background in this "technique". (A curious side-note is that the image didn't push down the content, as I'd have expected in normal circumstances, but went right over the top of it so it hid it (like z-index).
One thing that does bother me about this "technique" is that if you click on View Demo just below the code provided you will see on the working example page (forest background) the image clearly works as a background and the content sits happily on top! So I don't know whether the person providing the example was trying to mislead people or what! Or he got lazy and showed a different page that didn't use the exact code he provided. No matter.
In any case this leaves me with the job only half finished, as I still need a background image that works like the 'normal image' code provided.
On to Case 2 and on this page the first example provided gives the CSS for the background image - namely the code within html {...} part and also shows the same 'normal image' code as given in Case 1. In this case however, although the person provided a suggestion, the css and html doesn't really tie up properly plus some of the css and html seems a bit redundant. So this time when I used the 'html portion' of the CSS code (i.e. just the bit that was most relevant) I got two different behaviours in each browser and neither of which was quite what I'm looking for. In IE8 initially the page looks fine (background fills the page and content is on top) however when I zoom in or out the background also zooms in or out accordingly so is NOT fixed. In Firefox the background image DOES stay fixed, however because the image originally is not the whole height of the browser I'm guessing the code stretches it downwards (while keeping proportions - so the image essentially enlarges) to fill all of it. The quick way to get around it is to probably add some white space to the bottom of the image just to give it enough height that it doesn't stretch/resize automatically. But it would be nice to find out anyway how to get around this in the code. Overall I would say that the result in Firefox comes closest to the desired solution but of course it doesn't help matters with IE8.
Apologies for the lengthy description but that should at least provide plenty info for anybody that might have a possible solution for me. Essentially what would be great is if somebody could advise me how to take the code from Case 1 and apply it to a 'background' piece of coding like the type that's contained within html {...} in Case 2. Like I said, it's so nearly there but I just can't make it work atm after trying to combine the various bits of code this way and that... Alternatively, if somebody has another html-css version that works nicely for achieving this then please by all means let me know! Thanks v much in advance!

Answer 1

This works in modern CSS3 supporting browsers.  But not pre-IE9.
http://alt-web.com/TEST/Resizable-BG.shtml
Nancy O.