I haven’t been able to check whether or not the responsive images work properly, but the preview page shows it is (or at least that’s what it looks like)
Your code works correctly.
One thing you could do to simplify it, is removing the the second <source> in <picture>. When the browser window is bigger than 600px the browser will automatically select the fallback <img>.
Testing the responsive images
When you open the network tab in the Firefox DevTools you can see how the alternative images are downloaded when resizing the Glitch preview panel.
If you do the same in Chrome, be aware that Chrome won’t load the smaller images when it has already loaded the bigger versions.
Thank you for your comments and review, they are always appreciated (and help a lot)!
I will correct the code following your recommendation and test it using Firefox.