A ghost image is what one obtains when photographing an object using the Captcharoid Camera or drawing it using Jade's Pictionary Fetch Modus; it is useful for finding out the captcha codes of various items that are not handy or easy to captchalogue, such as those not in the immediate vicinity or those too large.
These ghost images have been extremely useful to the kids in a number of cases. Using her Pictionary modus, Jade was able to give Dave the codes for the Jumper Block Extension, Totem Lathe, Punch Designix, and Holopad, with which he modified his Alchemiter into a truly impressive machine, that Dave in turn did the same for Rose. Most notably, she was able to get the Holopad's code without expending the extraordinary amount of grist required to deploy it. However, where the Pictionary modus has the advantage of being able to capture the ghost image of anything so long as you know what it looks like, the Captcharoid Camera will always give you the code of anything you take a picture of. Jade's modus requires you to be able to draw it sufficiently well, or you may wind up with a ghost image drastically different from what you intended-- when she attempted to draw the Intellibeam Laserstation, she wound up with the ghost image of a gundam robot.