If you're in windowed mode, https://gyazo.com/captures works pretty well. It's a lightweight desktop application. You activate it and just drag a square over whatever you want to capture and it'll automatically upload it to their site.
(https://i.gyazo.com/d624cef0ee5fb277cd264cbdc7512770.png)
(https://i.gyazo.com/e9ac9baf7231ba6b8ca5828b37197a40.png)
The old ctrl+alt+printscreen and then paste into a new image or "paste from clipboard" into some sort of image program might work.