Image sizes and masks

I am trying to find my way in iBooks author, attempting to create a book with my editorial cartoons. Thus, there will be some texts interspersed with a lot of images and naturally, I want to have captions, too, especially since the book will be written in English, but the cartoons will have an original text in Swedish which will be translated in the captions.

But I have problems with the images sizes and the masks. It seems like I cannot resize the images freely. There is a minimum size and I cannot go below that. Neither can I edit the mask or simply remove it where it hides the images, cutting them off to the sides or the top. It doesn't seem to be very flexible that way. It would be good if I could edit the image size freely and individually and also the masks.

Any ideas what to do?