I use Spectacle for taking screenshots and I’ve enjoyed using it for several years. After that, I’d crop it in any image editor that has a freehand selector tool. Gimp would be the first thing I tried, but I’m sure you could find something easily enough.

Dumb question: do you have a key on your keyboard that disables the microphone? I don’t have my laptop in front of me, but I know there’s a key that disables the trackpad and another that disables the camera. I realized that I’d accidentally hit those by going through something similar to this.

Good luck.