This happened to me a while ago.
My Digital Rebel images started having the prefix _MG_ instead of IMG_.
I figured I'd run by some strange numbering threshhold. It was odd though. Sometimes I'd see IMG_ again ...
It bugged me.
I should have googled it long ago. Photo.net explains - this is what Canon does when you switch from sRGB to Adobe 1998 RGB color space. It's a JEITA specification mandate. I probably see IMG_ when I shoot JPEG instead of RAW.