CSS issue with image gallery

Hi everyone, new here ??

First off: I’ve been using wordpress for quite a bit now, including for my own costumers! And here goes my big THANKS to everyone envolved in it’s development, really a brilliant job (i earn a living out of it, so… i owe you alot!)

Second, what brings me here:
I’ve recently created a theme for a client (in somewhat of a rush, so, basically, it’s not validating at the moment), and installed the nextgen gallery plugin in it. All is working properly, except the CSS of it, which creates a funny/awekward footstep effect on the thumbnail row.
https://amprodesign.com/worldcup/media-gallery-2/photo-gallery?album=1&gallery=1

As you can see, i’m using one of the gallery’s css, but this happens in every css bundled with the plugin.
I’ve tested multiple options to try and fix this, such as removing the theme core css properties, the full gallery css, and the best i could get was a vertical list of the images. Funny thing is… it works as a charm under IE :S Guess that non-stardard browsers have some use after all!

Any help would be appreciated, and even rewarded if required (e.g., i’m willing to pay in order to fix this, since i’m running short on time here).

PS: Forgive me if i’m posting in the wrong forum, kinda noob arround the support forum!

EDIT: browsershot’s as well:
https://browsershots.org/https://amprodesign.com/worldcup/media-gallery-2/photo-gallery?album=1&gallery=1