No, 48x48 is not necessary. Statically link to the icon in desktop

configure.ac

@@ -77,8 +77,7 @@ AC_CONFIG_FILES([
     src/pam_panic_pw/man/Makefile
     src/pam_panic_config/Makefile
     src/pam_panic_config/applications/Makefile
-    src/pam_panic_config/icons/48x48/Makefile
-    src/pam_panic_config/icons/scalable/Makefile
+    src/pam_panic_config/icons/Makefile
     test/Makefile
 ])