Well, I can think of a couple o solutions to blown up images, but I'm not sure either are worthwhile IF there is a fix coming sometime soon-ish...
Have you tried putting the image at 100% width inside a div set to 70% or however much was your intended width?
Alternatively, just enlarge the image canvas to include exactly as much white space you want around it (so if you wanted it to be at 60% an the image is 360px wide, add 120 white pixels to it on each side).
Both these solutions require quite a bit of work, though, and any image intensive book will be a boring hell to adapt...
