Image Masks in WireframeApp are used to show parts of images selectively. For example, in image masking mode, dragging right side knob inwards will mask out right part of the image.
To do so, select the image element and choose Set Image Masking from the format panel. You will then enter Image masking mode, adjust the knob to show/hide parts of the image. Pressing Escape will end the image masking mode.