This is really the same question as http://www.mathworks.co.uk/matlabcentral/answers/39466-how-to-convert-a-img-file-to-dicom which you also posted your question as part of.
As in the responses to that question, we need to know what software produced the file, or we need documentation on the file format. The suffix '.img' is used by multiple vendors for different formats. There is no standard .img format.
