Oops, one more thing. Your .hdr/.img files must first be in AFNI format before you can run 3drefit:
3dANALYZEtoAFNI -prefix elvis elvis.hdr
3drefit -markers elvis+orig
(If you try 3drefit on your .hdr/.img files, you'll get the following error message:
e.g., ** 3drefit: Can't process ANALYZE datset elvis.hdr)
Peggy