Image names are not necessarily the same as image file names. When you attach an image to a drawing, AutoCAD uses the file name without the file extension as the image name. You can change the image name without affecting the name of the file.
The image file is represented by the ImageFile property on the Raster object. Changing the ImageFile property will change the image in the drawing. The image name is represented by the Name property, and changing the Name property will change the name of the image only, not the file associated with it.