Looks like maybe it was as simple as stripping out all those classes. (That'll teach me to use a converter, but I still am not quite at the DIY phase of epub creation.) Taking out the classes (which were apparently overriding all my "width="100%"" attempts) did the trick on the resizing:
<img alt="DSC04389.JPG" width="100%" name="Picture 1" src="../Images/tmp0_html_52416fa5.jpg" />
I would still welcome tips on how to make sure the image is centered. Right now, it's at the top of the page instead of the center. Thanks!!