Wonderful. You probably have "hide file extensions for known file types" checked. So when you downloaded the original, it ACTUALLY had a .img on the end of it already (which windows hid from you). Windows, unlike unix, cannot differentiate files based on their header, but instead relies on a useless extension in the file name of all places (ie, .img, .mp3 and .jpg at the end of the filename).
And noo problem mate, happy to help.
Device: Galaxy Nexus i9250
Previously: Nexus S i9023