Learn Swedish: Avataren är bara en bild på skärmen. - The avatar is just a picture on the screen.

You would place i sig directly after the noun it emphasizes: Avataren i sig är bara en bild... (The avatar in itself is just a picture...). As an abstract emphatic phrase, i sig means "per se" or "in itself," referring back to the third-person subject.

Why do we say "på skärmen" instead of "i skärmen"?

Swedish uses på (on) for digital displays, screens, and flat surfaces, matching English perfectly here. You watch things på tv (on TV), look på datorn (on the computer), and see images på skärmen (on the screen). Using i skärmen would imply the picture is physically embedded inside the glass or plastic of the monitor.

Avatar is a fairly modern loanword. How do we know to add -en to make it definite?

When English loanwords become established in Swedish, they are almost always assigned a grammatical gender—usually as en-words. Once it's an en-word, it follows standard Swedish conjugation rules, so en avatar simply becomes avataren (the avatar), just like en app becomes appen.

Could I use "endast" instead of "bara" to mean "just" or "only" in this sentence?

Yes, Avataren är endast en bild... is grammatically correct and means the exact same thing. However, endast is a bit more formal and characteristic of written language. For a standard or conversational statement like this, bara is the most natural choice.