You can mask and crop your images in Proto.io Editor.
In the Properties Inspector upon selecting an image, you will see a 'Mask/Crop' option. When you check that option, you will see the mask area, which you can edit. The mask area is not restricted to the original image.
You can also select the original image behind and edit that.
When clicking outside, the mask is applied and you see the resulting image. On this you change border, opacity, or apply filters and drop shadow.
Of course you can unmask the image by unchecking the option and also reset aspect ratio, which can be useful if you distorted the image while editing the mask. If you mask the same image again, the Editor 'remembers' your last mask and brings it back up.
How can this tool help you?
Save precious time by masking/cropping specific portions of your image; selectively showing what you want and style with color adjustments, filters, opacity, and drop shadows within the Proto.io editor.
Easily add borders of different color to your masked images. Give your mask a circular, square or rectangular shape and even change the border radius for rounded corners.
Bring your prototypes to life with animated masks. Use masks with transition animations to create impressive visual effects without leaving the Proto.io Editor.
You can also watch this video.