Thrown weapons are actually a spell. Not sure how to mod the graphics, but you can mod the value of the damage by setting up calculate environment
<Calculate InternalName="Value" ValueOwner="TargetUnit">
<Expression><![CDATA[[Unit_GetTroopCount] * 8]]></Expression>
</Calculate>
This will multiply the damage from each troop.
This is by far the easiest way to go about the change... there may be others using ranged abilities.