If you find the spellID/itemID from wowhead for the soul shard item and then use GetItemInfo to get the information you will be able to get the icon file used for that item.
http://www.wowhead.com/?item=6265
http://www.wowwiki.com/API_GetItemInfo
This means that 6265 is the itemID so using the following command will grab the icon path for you to use as a texture file name.
itemName, itemLink, itemRarity, itemLevel, itemMinLevel, itemType, itemSubType, itemStackCount,
itemEquipLoc, itemTexture = GetItemInfo(itemID)
To make things easier because you may not need the other info you can just do this.
itemTexture = select(10,GetItemInfo(itemID))
This will select the 10th item in the return list from that function.