By the way, for anyone from the future who's looking for the answer,
here's a post from Liz Castro, who suggests putting the image in a div and setting the div's width to get the desired size in iBooks (and then set the image to div size to avoid overflow outside of iBooks). I didn't manage to get this working, but for me it's not even an option since my image is in the middle of a sentence and can not be considered as a block like a div. "Currently,"
I learned on Jutoh documentation, "this won't work if you have multiple images or other content in a paragraph"...