If I hold a convex lens in front of my camera lens, will it magnify the image the camera sees, or reduce it?

If the object is less than or equal to the convex lens's focal length, it will allow the camera to focus closer than it otherwise would be able to. For example, if the object is at the focal length, focusing the camera lens at infinity will bring the object into focus on the film. And yes, the image:object magnification achievable can be greatly increased this way. It's a common technique for macro photography.

It really depends on where your object is.
If it is more than two focal lengths of that convex lens, it will reduce the image.

If it is between one and two focal lengths of the convex lens, it will magnify the image. This is the basic theory of how a "macro-lens" works.

It depends on the focal length of the lens and where you are situated relative to the focal length.

A convex lens converges the image to a point, so the image would appear smaller in front of the focal length, and it would appear upside down behind the focal length (and bigger or smaller depending on the distance from teh focal length)

I think when you hold a convex or any other type of lens in front of your camera will either obscure the image or/and distort the it.

You will magnify, a concave lens would reduce.

By placing a convex lens in front of a camera lens what you will do is to make the point of focus closer to the camera than before and unless you move the camera the image will be out of focus. If you move the camera closer to the object that you want to photograph to focus it then the in-focus mage will be magnified.

Cheers