Start at 0 and go up from there, rather than using -1. For instance, set the div containing your inputs and labels to a z-index of 100, and give the image you want to place behind it a z-index of 50.
Alternatively just set the image as the background-image of the div containing the inputs and labels. Since the image is probably illustrative and therefore presentational, it doesn't really need to be an actual img element.