Two thoughts:
If your map document has a coordinate system defined, any file with unknown units/coordinate system will be displayed using the map document's units. Check that the map document General tab shows units as unknown, and Coordinate System tab shows the Current Coordinate System as "No coordinate system."
Perhaps you need to go in and delete any pyramid layers or files created during your previous session. Go to the folder in a regular windows explorer/file browser, and delete any files with the same name, but different extensions than your original image. (ie: if your original image was picture.tif, delete picture.aux, picture.ovr, etc).
Regards,
Jim