Something on your server. This is what I see when loading one of the images. File not found.
http://cebulek.pl/render/w480/galleries ... terfly.jpg
Yet when I refresh the same page with. some random ?bah, it loads and processes fine:
http://cebulek.pl/render/w480/galleries ... ly.jpg?bah
Now, the same image works without adding the random parameters:
http://cebulek.pl/render/w480/galleries ... terfly.jpg
Some weird dysfunctional caching mechanism on your server.
That specific image now also loads in place in the gallery (for the specific size requested by my macbook 13" retina):
http://cebulek.pl/galleries/nature/
You really need to ask your host about this.
1. Open your browser developer tools > network inspector when loading a gallery, and see all the errors.
2. Then click to open on of the broken images in new tab, and see the error from hosting.
3. Then refresh the image with some random ?parameter at the end in url. This means nothing to X3, but apparently it seems to refresh the request on your server, bypassing some dysfunctional cache mechanism.
Surely your hosting should be able to figure this out.