It's a weird issue with wikipedia which had left me frustrated. When I go to wikipedia, with different browsers ( IE8, Chrome3, Opera10 ) no image in the site will show up. Even right-clicking the image to ( show, save, open in new tab/window ) will return nothing except when open in new tab/window I'll get a black horizontal line across the page. All my browsing across the web goes flawlessly, and every image in every site show up normally except in the wikipedia site. I have win7 ultimate with all updates.
Maybe there's something wrong with your hosts file? Open C:\Windows\System32\drivers\etc\hosts with notepad. What does it contain?
Otherwise try a different DNS provider, like Google or OpenDNS. Run the GRC DNS benchmark utility and change your DNS provider. That might help you access the images.