OK, here are the results of my comparisons between a KPF file containing the same image as an epub file(for upload to KF7, KF8 and KFX).
And @jhowell and Hitch are both right. When I compared these two images in KP3 they looked OK and quite passable. So my humble apologies for causing any unnecessary stress or spontaneous heart attacks about this. But yesterday, during my testing, I was getting poor images during test for the epub images when using my media query plugin. I guess I was doing something wrong in my preparation. I'm also quite relieved because this result means that my media query plugin for Sigil is OK for a few more years yet. I'm also not going to post the images for comparison, there seems to be no point now.
And, just for interest sake, here's the dual format code that is automatically produced for my epub images using my media query plugin for Sigil:
Code:
<p class="P38"><img alt="Word same as upload.jpg" class="mobionly" height="189px" src="../Images/image15.jpg" width="506px"/></p>
<p class="P38"><img alt="Word same as upload.jpg" class="kf8only" src="../Images/image15.jpg" style="width: 75%;height: auto;"/></p>
The plugin has been working well, without any apparent problems, for several years.