To wonder what where the boundaries of porn are?

[coconuttella]

For instance, an image of people with their boobs and bits covered up may be done highly erotically, yet I sense this wouldn't naturally be seen as porn, whereas an image of a nude may not be particularly sexual, but it would more readily be seen as porn.

Nudes aren't necessarily porn, because they haven't been displayed or done to sexually arouse. Which is why some art and statues aren't pornographic.

That's what used to decide if it was porn. That's how newspapers etc got away with topless pictures, they were thought of a bit of fun and not even soft porn.

We now grade images by sexual explicitness, it doesn't matter what reason they were done for.