First, AFAIK, iBooks ignores the width property altogether on the img tag.
But... I was finally able to reproduce what you're seeing by putting "width:100%" on the <body> tag! So maybe you're inadvertently doing something similar?? If so, then just use the default width -i.e. don't use the width property at all.
More details and a few example are at
http://empiricalepub.blogspot.com/20...ntortions.html
-Bill