WWW access Problem

When I use WWW API to access my server like this:

WWW www  = new WWW ("http://myserver.com/AssetBundles/cloth.png");
        yield return www;

-I cant get the image in Unity3D Editor(Webplayer, Standalone, ios) or Standalone.

this is the server log: "GET /AssetBundles/cloth.png HTTP/1.1" 403 - "-" "UnityPlayer/3.1.0f3 (http://unity3d.com)"

-But I can get the image in Webplayer Safari and iPod/iPhone

this is the server log: Webplayer Safari "GET /AssetBundles/cloth.png HTTP/1.1" 200 40805 "-" "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_6; en-us) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4"

iOS "GET /AssetBundles/cloth.png HTTP/1.1" 200 40805 "-" "Retrofits/1.0 CFNetwork/485.12.7 Darwin/10.4.0"

Any suggestions or solutions would be greatly appreciated.

Thanks!