You just gave the worst advice ever. Tell me how would you deal with a malicious PNG?
It depends on how the code is embedded. If it is the old double extension trick, then just enabling the ability to see the extensions in Windows will reveal that. If it using steganography, then you are not going to just stop that download, but that also requires an external program (malware/virus) to extract and build the final code from the image.
Stopping that malware/virus from ever getting there is pretty straight forward, but it also means giving up a lot of the default features of Windows. Example: Remove Java, Flash, and disable javascript, and activeX controls as a start. Disable all auto-downloads of everything.
Not to make it sound so trivial, as it is not. There are applications, such as Intuit's software, which require you to open your security up and expose your computer to every bad thing there is. Solution: Avoid those programs.
Then there are bank sites which also require you to drop all your security before they will work. I do not do business with those banks. They are all set to be hacked.
It takes more than just common sense to protect your computer from the bad stuff out there.
One last thing to consider. The more a system tries to do things for you, the more likely it will be hacked/infected. Most people need anti-xxxx software because most people are not willing to give up the flashy/sparkly stuff. Here is a test.
If you go to Youtube and the videos play without you having to do anything, then your computer is open to all manner of bad things. Get an anti-xxxx program,....quick.