feat: CSC-2799 - Update preview image for alien rpg

.gitignore

@@ -150,4 +150,6 @@ sync_sheets.sh
 
 **/*/dist/*.html
 **/*/dist/*.css
-**/*/dist/*.json
+**/*/dist/*.json
+
+.idea

alien_rpg/sheet.json

@@ -3,7 +3,7 @@
 	"css": "alien_rpg.css",
 	"authors": ["Medieve","Scott Casey","Sigil Services"],
 	"roll20userid": ["23472","459831"],
-	"preview": "alien_rpgpreview.jpg",
+	"preview": "alien_rpgpreview.png",
 	"instructions": "This is the official Alien RPG Roll20 character sheet. It includes drag and drop integration with the [Alien RPG Compendium](https://app.roll20.net/compendium/Alien%20RPG).",
 	"compendium":"Alien RPG"
-}
+}