I got bored yesterday, so I wrote up a quick ASP script to pull item images from the RuneScape database based on Item IDs (the unique ID assigned to every Runescape item).
This program could allow us to see new tradeable items that are added into the database before they're officially released in Runescape. It also brought to light the interesting way that Jagex assigns ItemIDs to items - they seem to use alternating IDs (skipping the odd numbers) for certain groups of items, and completely random placement for others.
The newer items seem to have the highest ItemIDs (like the new shields, Fist of Guthix items, crayfish, top hat, etc), even though there's plenty of free ItemIDs in the lower numbers.
My program scans the ItemIDs from 1-10,000 on one page, and 10,000 to 20,000 on the other. I tried ItemIDs below zero and up to 40,000, but I couldn't find any "hidden" items.
I already noticed a graphical glitch in the items #1716 and #1718 (unblessed and holy symbols), which apparently use the same sprite.
How to use:
Simply open the link to either one of the pages, and allow several minutes to load. The program will attempt to pull images for each ItemID from the RS database, any unused ItemIDs will will be displayed as a broken image "Red X".
Please allow several minutes for the pages to load.
If anyone wants the code, I'll be happy to post it (it's only about five lines). Enjoy!