[Plugin: NextGEN Gallery] Distorted images with non-flash slideshow

Hi guys,
I just discovered this plugin some days ago and it really looks great. Unfortunately I have problems to get it to work correctly as some images get distorted all the time:
https://www.snn-nachhilfe.ch/?page_id=262

I tried all different settings (as there aren’t so much for javascript slideshows) but the problem persits. I also resized the images manually so that they fit without additional resizing (max. 500×300) but still they don’t look right. The interesting thing is that the visible image resolution sometimes is correct but the image shown is still distored as not all parts from it are shown (image 2 and 4 for example).

Any idea? I already uploaded all images multiple times and I also cleared cache.

Thanks in advance.