Would it be possible to just build the Title page entirely in SVG?
I was doing some messing around with exporting an SVG title page right from Inkscape... The pure text version worked fine, but adding the image brought in some odditities (this will require more testing to try to come up with a step-by-step process).
I also stumbled upon this topic from pdurrant:
https://www.mobileread.com/forums/showthread.php?t=55563
Might be able to use that code as a very good base to build from.
I plopped your PNG image into pdurrant's code, tweaked a few of the numbers to get it centered, and it showed up fine in Sigil... BUT, when I tested on my Nook (also ADE), all I have is a blank space where the image should be.
Edit: I was able to get your image to show up using the Inkscape method (it embedded the image as Base64 I believe). The Inkscape method was able to show the image in ADE fine, but on my Nook, the placement of the image was completely wrong.