Edit and adjust images from right inside Google Slides
Sometimes to get an image just right in a presentation, you need to make some small tweaks. To help you do this without leaving Slides, Google has made it possible to crop and add borders and they have recently added new editing options. You can now select "Image optons" from the toolbar, format menu, or right-click menu, where you can adjust the opacity, brightness, and contrast of an image, or recolor it to match the theme of your presentation.
If you haven't tried out Google Slides yet - please do. I know you won't be disappointed.