I've never had success using a background image for just a page with overlay text thereon, for my .imp ebooks or conversions.
It's usually the background image first, then followed
by the text, not on top
of the image.
It's all about positioning and when rendering html in a top-down design, that linear nature seems to be the only model understood by our ebook creating programs.
Sure, you can try absolute positioning. That's the only way I've seen .pdf conversions to .html done (visually) accurately (with text), but it's a whole new ball game for ebooks creating programs to render and match. I think this is one of the reasons .pdf to .html never converts layouts well.
Been there and now use PDFRead when I can't win!
Just my two cents.