Pictures in Lightbox appear too small (resized?)

Hi there!

I have two little problems on which I can’t find the answers:
when I klick on a thumnail the “lightbox” opens, in this lightbox the vertical pictures appear too small (I upload them with 533 x 800 px, but the size they are shown is way much smaller) and the horizontal pictures look a bit blurry even though the original is very sharp.

It seems as the theme resizes the images somehow?

Any idea how I can fix this?

I tried AJAX already, but the thumbnails are not the problem, neither the “swinging lightbox”, so obviously it couldn’t work 🙁
the “problem-hp”: http://www.picturemyworld.de

Thanks in advance!

Best regards,
Tina