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.

I inserted the images by clicking the "image" button on the right, and chosing "thumbnail". Actually I did not know if the forum software creates a scaled down image file and shows this or if this scaling down is done by the client browser, until you pointed out that fact.

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;".