You can CROP AN IMAGE TO A SHAPE in Google Slides just by clicking a little down arrow next to the Crop symbol.
Don't overlook that little down arrow next to the Crop symbol in Google Slides! When you insert an image and click on the little down arrow, you'll have lots of choices for cropping your image. Right at the top is Shapes but you can also choose Arrows, Callouts, and Equation. When you hover over any of these categories, you'll see all of the options for that category. The Shapes category definitely has the most choices. After you click on a shape, the image will immediately be cropped to that shape. If you want to edit the crop, click on the shape and then click on the Crop symbol, not the down arrow. Small "handles" will appear on the sides and the corners of the image. Use the "handles" to edit the crop by clicking and dragging to zoom in or zoom out on the image. Once you've edited the crop, you can resize the image until it's just the way you want it.