Download this: Tom's Cache Suite
and this http://i.imgur.com/6QT6enz.png
Load your cache, View/Edit Image Archives -> Media -> navigate to the archive named 'mapfunction.dat' and either click the + and pack a new sprite or replace any existing blank one. Remember this id, as you will use it later.
Follow this tutorial: http://www.rune-server.org/runescape...map-icons.html
Code:
mapIcon = new Sprite(mediaArchive, "mapfunction", 5);
Replace the number indicated in red with the sprite index that you memorized from earlier.