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.

But the full image has to be transported from the server to the client. More efficient would be to use a thumbnail already existing on the server; this saves transmission capacity and time, and computing time in the client.

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.

That can't be completely correct, since this stair only occured with the third image, not the second. The height of the DIV or IMG including the margin should not be the problem.

But it might be that I had one space character or what else too much between the two images.