Same here. Until today, I wasn't even aware that there was an "official" pronunciation for .PNG. "Ping" just sounds stupid if you're not referring to pinging a server, imo."Ping"? Ping is something you do to servers to see if they're still alive while you try to remember where in your house you stashed it.
I've never heard someone use it to refer to .png file type, and have only ever heard it as the acronym spelled out: P. N. G.
I seem to be falling in with the consensus here. Given that .JPG images often use the extension .JPEG instead (with an actual vowel and everything) I've always pronounced either way as "jay-peg" (and "em-peg" for .mpeg formats). .GIF is a tough one for me to nail down my personal preference, since I've used both pronunciations at various times in my life. I think I tend more toward "jif" though, even though I feel slightly stupid while saying it. (To be fair, I feel slightly stupid while trying to pronounce any of these.)
.BMP I just refer to as "that thing which shall not be named" and try to forget it even exists. Never had any use whatsoever for it.