What I suggest to you is to get the URL for a button image from the fiddler and execute it in a separate browser.
Then please check if you have the mapping for _wpresources in your IIS
Manager. I attached a screen shot with how your web directory is
supposed to look like.
Afterwords confirm that you have the files in the \Program Files\Common
Files\Microsoft Shared\web server
Also please inform us if the Default skin is working (set the following code <property name="Skin">Default</property>
in the respective ConfigFile.xml).
All the best,
the Telerik team