[plugin: jquery colorbox OR nextgen gallery] wrong position of images

I’m starting to work with WordPress and I have a problem with lightbox-like plugins.

After clicking the images the lightbox appears at the bottom of the page instead of its center. What’s weird is that it works OK with Internet Explorer 10 but not with others (Firefox, Chrome).

Here’s how it looks currently with Nextgen Gallery plugin:
https://przygarnij.e-net24.pl/uploads/problem11.jpg
Jquery Colorbox appears exactly the same.

Here is another test website of mine which has a similar problem, only with the lightbox appearing way below the viewable area:
https://przygarnij.e-net24.pl/wp3/jakis-tytul/

What am I doing wrong? After switching the theme to any of the default ones the lightboxes work as intended.