Is there a way to keep the quality of pictures, when zooming in with e.g a camera?

When the distance from a 3D layer to a Comp Camera is equal to the Zoom value  of the camera (press 'aa' to see it), the layer's effective scale is 100%. Move the camera closer to the layer, and the effective scale increases. 

If you want to push into an image or the footage to fill the frame with 1/2 of the image, the image needs to be twice as big in pixel dimensions as the comp. That means it needs 8 times as many pixels. Some plugins do a little better job of scaling up layers so that you can get away with an effective scale value of about 150 to 170%, but that's about it. After that, you need to rely on motion blur or some other effect to camouflage the pixels and make them look like some kind of transition.

There's nothing there to "keep the resolution". That's just how pixel images work If you really want more details, you have to create a larger version in the first place, be it just a doctored up version from Photoshop. Other than that only vector-based content can be kept sharp under certain conditions using continuous rasterization, but this has its own caveats due to how it's handled in AE.

Mylenium