I used teachable machine (https://teachablemachine.withgoogle.com/) because I have no idea how to actually do machine learning.
I ended up using about 800 pictures, split between containing an armpit and not containing an armpit.
I also hacked together some custom python and Arduino code to communicate over serial and send whether it sees an armpit.
This was my first real robotics project, and my first ML project. I thought it was a lot of fun, and I learned a lot. Hopefully it inspires more dumb robotics projects. Because honestly, those are the best kind.
I have never had such a high ratio of the word armpit in a piece of writing before...