To find the EPSG code for an image, run the command line tool mrsidgeoinfo, which is located by default in <Express Server installation directory>\ImageServer\bin.
For example, you might enter the following command:
mrsidgeoinfo -epsg <file name>
If an EPSG code is returned, use it (see Specifying an EPSG code). If no EPSG code is returned, then run:
mrsidgeoinfo -wkt <file name>
The resulting well known text string (WKT) may be useful in finding the corresponding EPSG code.