It's possible to make basic edits to images using the tools that exist in Google Slides. Once you have uploaded an image, you can recolor it, adjust its brightness, contrast, and transparency. A recent update allows you to add drop shadows and reflections. You can also crop and mask pictures. Masking an image allows you to change it into a different shape which means you can be even more creative with your images.
This video shows you how to make a few of these edits. Please note there is no sound on this video.