Google Slides is basic but effective. I honestly love a lot of Keynote features, but Google Slides makes it easy to share and collaborate. There's a few things that are tricky to do though!
Add an Image Mask (in Google Slides)
This is different from cropping - everyone can do that.
- Select your image.
- Click on Help > and search for "Mask image: Oval". (note: there are several other shape options like stars etc)
- Click into the image to adjust the dimensions of the mask and image below to get the desired shape.
- Google Slides - official site, where you can create your own slides.