Can a image tag in html z-index behind a css background image?

Sure, a HTML element with a background-image and a higher z-index can overlay any other HTML element, in your case a simple <img /> tag. But the <img> tag has not to be nested within the tag holding the background image.

This will work

<img src="../image.jpg" />
<div style="background: url(../image2.gif); z-index: 1">Content</div>

this not:

<div style="background: url(../image2.gif); z-index: 1">
    <img src="../imag.jpg" />
</div>

It works if you set position to either relative or absolute. Be sure to set it as it may just be inherited.