Well, for us, we had just went ahead and simply changed our Magento site's backend setting to use HTTPS on for non-secure pages etc. as well, just a blanket "Everything" (System->Configuration->(under 'GENERAL' tab)->Web->'Unsecure' setting) -----
----- After the change it passes all validation as images being served to the M2e gallery as well are all https-based by default now. Figuring since seemingly not just the upcoming and ongoing changes for Google Chrome browser itself (https://security.googleblog.com/2016/09/moving-towards-more-secure-web.html), but the web generally as a whole are/eventually will all be going this route, might as well just opt for https-ing it for all pages/media etc. That was our decision/solution for it all.
If any of you out there choose this simple and very easy route you can of course still trigger the Automatic (or Manual, depending on your Sync settings) updates for all your M2e listings by making any kind of random/insignificant change to your master Listing/Description template(s). After you had of course made the quick Magento http-to-https backend change noted above first, it will then convert everything over, including for the M2e image galleries. Perfect. The guys here at M2e may just simply release an official update that addresses this, but, again, looking at everything else beyond ebay/Google etc. here currently, we just simply went ahead and did it all this way permanently moving forward instead.