spritesheet problem: makes characters float above ground

Hi,

I'm totally new to Unity, so 'hi' to everybody. I have a strange problem in a simple 2d platformer I'm trying to make.

When I check in the editor, my scene looks like this:

Nothing wrong there right. Well once I press the play button this happens: The zombie suddenly floats in the air. I first thought it had something to do with the 2d box collider. But that wasn't it. It's really weird, as you can see that the origin point of the zombie seems to be confused. So I tracked back, and I think the issue has to do with my sprite sheet. But I can't figure out what the problem is. In the above example, the original point was set to the bottom left corner.

Now when I change the alignment to for example 'center', then the zombie floats less above the ground.

Does anybody know what is causing this issue or what I'm doing wrong. Thanks very much in advance!

cheers, Andreas