For what it may be worth, Jon, those dual images are very common in Vellum generated ePubs. One is for .azw3. The other (generally much smaller one) is for .mobi ebooks with the image size defined in pixels. The original code which you seem to have stripped.
For instance this is a very common item in a .azw3 file converted to ePub (please note the data-AmznRemoved="mobi7" and data-AmznRemoved-M8="true" items):
Code:
<div class="publisher-block">
<div class="title-page-publisher-logo-block title-page-publisher-logo-aspect-wide">
<div class="title-page-publisher-logo-size-container publisher-logo-width-1000-pad-65-1000">
<a class="link-contains-image" href="http://www.whoknows.com"><img src="../Images/image01888.jpeg" alt="WhoKnows Publishing" class="title-page-publisher-logo-image high-res" data-AmznRemoved="mobi7"/>
<img height="75" alt="WhoKnows Publishing" class="title-page-publisher-logo-image low-res" src="../Images/image01887.jpeg" width="172" data-AmznRemoved-M8="true"/></a>
</div>
</div>
</div>
You will also find Vellum using similar code for scene break images where the .azw3 and .mobi images are different in size and, as usual, mobi requires the size to be in pixels.
Also note that KindleUnpack leaves those items present whereas a calibre .azw3 to ePub conversion removes them.
And for your query elsewhere... Really? Have you never bulk renamed files in either Sigil or calibre's ebook-editor.