The idea of masking an image on your slide is to fit the image within a given shape, hiding the excess parts of the image behind a ‘mask’.
To achieve this in your Google Drive presentation , first open your presentation and select the image in your chosen slide:
Now click the dropdown arrow next to the Crop button, and select a shaped mask from one of the categories you’re offered.
Your shape will appear superimposed on the image, where you can resize it by dragging the handles:
When you’re happy with the result, click Enter to apply the image mask.
To place an outline around your mask, use the Line tool from the toolbar while the mask is selected.