This looks like it has been injected on your server because of weaknesses in your server's and/or web application's and/or FTP security.
Here's some more really helpful information on the imgxxx.net malware hack and how to check the extent of infection and steps to remove and protect against.
(variations might contain url imgaaa.net,imgbbb.net,imgccc.net,imgddd.net)
Do you have anti-virus software installed? Do you have file upload capability in your site that is publically accessible? Have you altered the permissions to any folders in your website beyond those set by default by IIS or Apache?
Running Anti-virus software on your server is essential (try Microsoft Security Essentials 2 for initial protection). Else products like F-Prot, Nod32 and McAfee.
Review your security policies on RDP and FTP access and if possible restrict access to specific IP addresses.
And then also look at securing your application from Cross Site Scripting (XSS) and SQL injection vunerabilities. Products like URLScan 3 and products by companies like Port80 software are good too.