Yes Dave! That solved my problem!
That is a little sneaky of Atto
So I was checking the "Auto Size" box to proportionally change the image length when I changed the width. The trick is (as you pointed out) to then 'uncheck' that box right before I save.
Yes, you are correct -- when you go back to edit the image again, the box will again be checked, so one needs to un-check it before saving. This would be nice to add into the docs somewhere.
Thanks for helping me with this!