Add documentation for no_png keyword

dictionary.md

@@ -475,6 +475,11 @@ WARNING! Options in this section may change without notice, and should never be
   -*Dependency*: `export_images` must be set to 1 in order for the keyword to take effect.  <br />
   -*Turn off/on*: 0/1 <br />
   -*Default*: 0 <br />
+
+**no_png**: do not export any pngs (including standard images and images of calibration solutions). <br />
+  -*Dependency*: `export_images` must be set to 1 in order for the keyword to take effect. <br />
+  -*Turn off/on*: 0/1 <br />
+  -*Default*: 0 <br />
 
 **no_png**: do not export any pngs (this includes images and plots calibration solutions) <br />
   -*Needs updating*: might be better to change the logic (avoid the double negative) !Q. <br />