To Urbannature: Thank you for checking of the working of my testing galleries! Hope, it will helpfully for understanding - where are the problems with galleries. Maybe, the problem is with the server's side too?
Actually, in my experience some flash-galleries work well with 100+ images (like on the flickr, for example) for introducing of a big-size' photo-sets to most of on-line visitors.
I have checked your gallery once again in IE9. The link of your gallery is http://www.urbannature.nl/?portfolio=trouwen#!prettyPhoto . Please, make the link as permalink: http://www.urbannature.nl/portfolio_item/portfolio_trouwen/ or something likes that in admin panel - maybe, it can help to repair the problem. It is only small idea about.
The other idea is: please, type some words into the html-section in your admin-panel (into editor's field of gallery's page). The text will be appear above of pictures in your gallery. Maybe, it also can help.
BTW, what are the names of images in your gallery? May be, the names contain some "!", "?", "é" or something likes that? Just asking about.
And yes, you right about of the light box in your gallery - now the size of your photos is too small. Actually, the flickr's standard is 1024 pxs, but the users can use the size 1270 pxs too in order to provide the best possible quality inside of the slideshow section there (of course, each of users wants to increase the resolution of uploaded images for the slideshow - for example, to 1500, but now 1270 pxs size is maximal limit there for playing photos without of resizing). I suppose, you may use some 1024 pxs size around for your gallery in order to provide the comfortable viewing for visitors.