images look strange

I just uploaded a few pictures and noticed, that they look kinda strange.

What I did:

I use an SBIG ST-7 CCD-Cam (9µm pixel-size). My telescope (10" Newton) has a focal length of 1500mm.
Therefore I should have 1.23"/pixel. The help page of VPhot told me, that the ST-7 should have a gain of 2.3 e/ADU.
This ist what I wrote in the telescope-setup-page.

The tracking of our telescope is not very good yet, so that most of my stars look more like ovals than circles. We will work on this issue.

When I then view my images, they look strange. Also the histogram looks strange.  To even see my stars, I have to change the values in the image display tool to like 100 and 500. When I view my stars with the pixel/ADU mode, there are a view pixels inside this star which contain nothing, the value says "0". I attached both, the histogram and a star-picture to show what I mean.

Also, the values shown for the content of the other pixels are strange. When I view my images with other program like Astroart, they show me values
of a view hundred, not over 30000 like in VPhot. Astroart also does not show me "holes" inside my star image.

Despite those viewing problems, the photometry seems to work. But the FWHM-value for each star differs very much from the value I get out of Astroart. There, I get usually about the same value for each star.

I hope somebody can give me a hint, what is going wrong here.

greetings

Katy