remove mapImage property

lib/Properties.hs

@@ -85,7 +85,7 @@ checkMap = do
     $ \cols -> warn $ "collisions between entry and / or exit layers: " <> prettyprint cols
 
   let missingMetaInfo =
-        ["mapName","mapDescription","mapLink","mapImage"]
+        ["mapName","mapDescription","mapLink"]
         \\ fmap getName (getProperties tiledmap)
 
   unless (null missingMetaInfo)
@@ -108,7 +108,6 @@ checkMapProperty p@(Property name _) = case name of
   "mapDescription" -> naiveEscapeProperty p
   "mapCopyright" -> naiveEscapeProperty p
   "mapLink" -> pure ()
-  "mapImage" -> pure ()
   -- usually the linter will complain if names aren't in their
   -- "canonical" form, but allowing that here so that multiple
   -- scripts can be used by one map