Fixup conditionally used params (#1076)

src/image.cpp

@@ -990,8 +990,9 @@ void Image::compute_SSAO()
 
 /* ---------------------------------------------------------------------- */
 
-void Image::write_JPG(FILE */*fp*/)
+void Image::write_JPG(FILE *fp)
 {
+  (void)(fp); // suppress unused parameter warning
 #ifdef LAMMPS_JPEG
   struct jpeg_compress_struct cinfo;
   struct jpeg_error_mgr jerr;
@@ -1022,8 +1023,9 @@ void Image::write_JPG(FILE */*fp*/)
 
 /* ---------------------------------------------------------------------- */
 
-void Image::write_PNG(FILE */*fp*/)
+void Image::write_PNG(FILE *fp)
 {
+  (void)(fp); // suppress unused parameter warning
 #ifdef LAMMPS_PNG
   png_structp png_ptr;
   png_infop info_ptr;