A Trojan Downloader is a malicious program typically installed through an exploit or some other deceptive means such as an Email attachment or Image download that allows the download to install malware onto a victim’s computer.
Trojan Downloaders are typically installed without user interaction through security exploits, and can severely compromise system security.
Typically site’s coding or images will become compromised through weak security, and allow for that code or image file to be re-written with malicious coding which when visited by an unsuspecting visitor, is downloaded onto their computer and is then infected.
The best protection against these types of Trojans is to ensure that your computer is fully up to date with the latest Security Updates and Patches. Also, ensure that you have Anti-Virus software and a Firewall configured to protect your computer. It is wise to schedule a full system scan of your computer with Anti-Virus software at least once a month with scheduled automatic updates.