What I did:

I added a picture to a score (Ancored at some horizontalFrame) and change it's Stack/Z-order to 0 so it is show behind anything. In Musecore that works and the picture is shown behind the stafflines.
But when I upload it to musescore.org and view it on the website it'll look like this:

^- Stafflines are behind the picture.

Analysis:

Well the what the musescore.org webserver does is in the rendering the *.mscz with musescore 3 some *.svg that is later embed and show on the users website.
So I got the idea to try export to SVG. And yes at the exported svg there is the same problem.
To check I opened/tested the produced SVG in Opera, Firefox and Inkscape .
All with the same result: Stafflines are behind the picture!

Deeper look

Well Inkscape has a nice feature called XML-Editor:

Well XML-Editor shows that the SVG:images (image78, image80...) [the pictures] are written after the SVG:polylines [Stafflines].

Changing the order so that the SVG:images are in the first place fixed the problem.

=> Well there is probably some logical bug in the Musescore code that dumps all the Stafflines before any other ordered objects.