Some test results on the 'test image auto scale SOURCES.zip' in the first post above.
I added two things to your testbed, with unexpected results:
1. I increased your header by adding 5 <hr>'s to see if eBook Publisher would accomodate this header. The answer is NO! I had expected/wished this wasn't so.
2. I increased your image size by 300% and added them as a third set of images. I had expected this to 'break' the 1150 or 1200 but they appear to be the same as the second set of images. Using the same aspect ratio causes them to shrink down proportionately and thus not be a problem.
Then I changed the aspect ratio and made them 300% taller, but keep them at 200% wide. I, instead, used these re-sized pictures as the third set of images. This then did break as the taller pictures bled into the bottom margin again! There's no winning situation here!
Attached are my results!