Opera is based on Chromium, so it uses the same developer tools as Chrome does. There are lots of tutorials for Chrome's dev tools. See https://www.google.com/search?q=google+chrome+developer+tools+tutorial for example. They should help with Opera too since it uses the same thing.
@leocg Thank you for the answer.
Sorry, but I have the software in Italian. Let's see... I can store default URLs, to be opened when I launch Opera. I would also like to be able to indicate the context in which they are to be opened. The context are those customizable areas in the side bar, the first item from the top. I am attaching a screenshot to understand. !
That's normal behavior. Save as and save image as override that setting.
For it to work with an image, you'd have to left-click on a link to an image to navigate to it where the server sends the image with a Content-Disposition of attachment to tell the browser to download it instead of displaying it.
You could have permission issues with your user data and cache folders. You could rename them to start fresh in Opera. Could work even better than just a reset of Opera.
If you're using Opera Sync, you can stop syncing and log out of your Opera account to test if the issue goes away.