I found that note in image.php of zenpage default theme. In thickbox.css I deleted the line:
*{padding: 0; margin: 0;}

Now I don't need anymore the extra margin-top: 16px; rule.
I've not experienced any changhe in the behavior of Thickbox visualization.

It was actually not a issue of the thickbox but on the image page itself where it cause a wrong top margin. At least with the browsers I tried when I added that.

Yes, I have understood this. I wanted to point out that removing the line I posted above from thickbox.css, the issue on the image page is solved..

Ok, I will test that, it could be browser specific..:-)