Hello

As first: there are no thumbnails in your posting. You embedded the original images from wikimedia and the BB-Code says it should be displayed as thumbnails. But it uses the original image in it's original size.

Yes, but scaling it down. Image info by my browser (Firefox) says:
1.024px × 768px (Skaliert zu 150px × 113px)

This scaling down is performed by the client (the browser), not by the host (the forum), since otherwise the browser would not know about the actual image size and the size of the scaled down image.

Yes it's the browser that scaling it down at rendering time. So there is only a large image shown with small height and length.

I am not sure what the "left" and "right" paramter to the "image" do -- my guess is that this leads to some CSS code like "float: left;" and "float: right;".

That's absolutely correct. And that's the reason for the staireffect of the side by side shown images. With float: left; comes a margin-top and that's added for every image.

Tschö, Auge