As I mentioned already, if your problem affects only thumbnails hack, there is only a way to remove the image width/height attributes so all images are displayed with their original size. For old thumbnails posted in threads new version should work taking the old thumbnail sizes, f.e. if you check in
this thread, you'll see there are old-type thumbnails resized proportionally and new-type thumbnails having static width and height as in the new version of the add-on.
I don't see much sense in reworking on that add-on and I think we'll keep the current schema when all thumbnails created are JPEGs and all they have static size set in options. This is more suitable.
As about this option -
if the entire pic could be shown proportionally using a narrower width, it would be narrower, but never wider than the max setting- it was implemented in the previous versions of the add-on and as I mentioned, it brings more troubles in handling the images else we may expect, and it's a "difficult" way - currently we have much more simpler and affordable way.