Looking at the calibre Job details for the DOCX -> EPUB conversion gives the following clue
Code:
Detected an image that looks like a cover
I'm far from expert in calibre code but a quick look at some of the DOCX cleanup code shows that calibre does some basic tests to try to decide whether the first image in the source document may be a cover, including:- checking the HTML for the page where the image appears to see how many 'other' tags precede the <img src="first_image" /> tag, i.e. is the image very near the top of the page.
- is image height/width ratio between 0.8 - 1.8
- is image height x width > 160000, i.e is it a large image
It looks like your logo image may be (at least???) 1500x1767 which is pretty big for a logo. I suggest adding your logo as a much smaller image and trying the conversion again. I don't have a new enough version of MSWord to try it myself.
In the long run you'd need to discuss the situation with Kovid. I don't know how the first-image-might-be-a-cover rules were originally decided and whether they could be changed.