Most programs let you crop an image by choosing its selection handles (sometimes you need to choose a cropping tool first) and move those handles into the image. Whatever appears outside the selection frame when you release the handles is cropped out. But iBooks Author doesn’t work that way: If you pull in a selection handle for a selected image, iBooks Author resizes that image.

To crop an image in iBooks Author, you use the masking tool by following these steps:

  1. Select an image.

  2. Choose Format→Image Mask.

    Alternatively, press Shift+Command+M. The masking controls appear:

    • Move the selection handles. Determine the size of the image to keep by dragging the mask’s selection handles to the desired positions. The shape and size of the image are determined by these handles’ positions.

    • Drag the image. Drag an image within its mask to determine what portion of the image is retained. Positioning the selection handles usually accomplishes this goal, but you can refine the portions displayed or change it altogether by dragging the image.

    • Use the slider. Adjust the position of the slider to make the image larger or smaller within the mask.

    You use these controls to determine what portion of the image appears in the e-book (and in iBooks Author) and in what shape and size.

  3. Click Edit Mask to apply the masking.

    The masking tool doesn’t disappear, so you can still change the size of the image, but you can’t adjust its selection handles (they now scale the image) or drag the image within the mask to change what portion is shown. To make those changes, repeat Step 2.

  4. To undo your masking, select the image and choose Format→Unmask.

    Or press Shift+Command+M.

You can also apply nonrectangular masks to an image. Choose Image→Mask with Shape, and then choose one of the 12 options from the submenu. These masks work like the standard rectangular shape, which happens to be the first option in the submenu.