aliasing in image processing
i see aliasing happening when the image is captured with a sensor that cannot capture higher frequencies.

Exactly!

If i capture a image, is it always aliased?

Not when the image content is sufficiently bandlimited.
That's often the case because of the physics behind things, or simply because the motive isn't higher in frequency.
(Physical effects include motion blur, abberation, depth of field/ring of confusion/bokeh, wave effects on apperture edges, lens imperfections, air density variations and many more. I'd recommend you look for tutorials on how to take really sharp macro and tele photos to learn what effects can reduce your image bandwidth.)
But, sure. Go photograph a fine grid, and you see a Moiré pattern. By the way, your eyes have limited resolution, too, although I'd presume the brain largely adapts that away.

Other types of aliasing are as far as i understand not really aliasing. e.g if a screen has not many pixels, the image is aliased.

You call it aliasing, it aliases things, you describe it as aliasing: It's really aliasing.