Recently, an image was downloaded from another Xara forum site and posted here on TalkGraphics.

I think the intention of the member who posted the image was to illustrate a question by another member who posted his question in both forums.

To the best of my knowledge, the member did not ask permission to download the image. He just downloaded it from the other site and posted it here to better illustrate the member's question. And to be honest, I am sure he never even thought that this might not be OK.

So, why is this wrong?

It's wrong just as taking something that does not belong to you is wrong. Your cell phone, your sun glasses, your car, your credit card, your identity. While the value of an image posted on the web may not be as great as these items in monetary terms, the image belongs to someone else. Before you download any image that is not your own, ask first. It's not only polite, it's the right thing to do.