Just click on the extension and search for what you're looking for, then drag the image to the gray comments area on a Slide Deck. (Note: this works for Google Docs too, but the images won't go into the gray comments area--you have to add them directly into the document. There are many Bitmoji options for student feedback that are good, but you will find that some of the choices are not school appropriate... If you need to crop out part of the image, such as a word you don't want to use, just double click on the image and use the crop marks.
Your students will enjoy seeing your animated comments. Of course, you can also add text comments too if you need to say something more specific. The Chrome extension makes it easy to drag your avatar into emails. On a Mac, you can copy and paste into the Messages app.
I have even used Bitmoji in a Google Form (right click on your Bitmoji selection and click "Copy Image Address," click the "Add Image" icon in Google Forms and then paste the URL. See just a part of a question and two of the answers with pictures below: