Add test for larger image size

It was discovered that large images would cause rendered pages to be
blank. This test ensures that larger images, though still reasonable in
size, are rendered properly.

Author: cspray

test/fixture/docbook/large-image.jpeg

@@ -66,3 +66,9 @@ They are hand drawn, that's why they look rubbish.
 ## Inline PUML file
 
 ![An external PlantUML diagram](./external-diagram.puml)
+
+### A larger image
+
+This is an image that's over 500kb so we can be sure that bigger images work:
+
+![A snowy mountain](./large-image.jpeg)