Google Slides has been updated for the better. Recently Google released a new Google Slides option for editing the images in your slides. While in the past, you could re-size images in your slides, now when you add an image to a slide you can crop it, apply borders to it, and apply masks to it. To do any of these things simply select the image then select an editing option from the new image editing menu. See the screenshot below for the location of the new image editing menu. Click here for Google's post with directions on using the new image editing tool.