Well, are the images in your link and the one above the same image? If so, when I look at the one in your link (which does look better by the way), I see that it is twice the file size (28K) and 720x90 pixels vrs. the one above which is 14K and 468x60 pixels in size. If those are the same files, then yes, this site compresses on uploaded files. They may not on linked images, but most forum software I know about gives the option to compress uploaded images.
Let's see if this link to your other image works and how it looks:
Does it look to you the same as it does on the other site or like the one you posted above?
Mike
P.S. I mean nothing against this site if they do compress uploaded images. That's the way it is on almost all forums and is a feature that helps keep a site from bogging down from files that are two big. It's their disk space and bandwidth after all.