Sorry, but no.

Firstly PNG and BMP files can contain _any_ binary data - including executable data (this has been shown a number of times - remember the shldocvw.dll vulnerability in Windows at the end of 2006?)

If an exploit is found that allows execution of arbitrary code then it doesn't follow that you'll get a garbled image - instead you may get a partially rendered image with the remaining binary data escaping into userspace to cause whatever damage it is capable of.

Windows users using limited user accounts should be less vulnerable to this form of exploit but that doesn't mean there is nothing to worry about.