The individual moons of Jupiter can be identified using their relative brightnesses. If you have an image which contains all four Galilean moons then the order from brightest to dimmest is: Ganymede, Io, Europa and Callisto. In Nikki Barron's composite image, Callisto is missing because it is out of the field of view. See if you can use this information to determine which path on the image belongs to which moon. A word of caution in using this criterion: when the image has been taken using a blue filter, then it distorts the color and brightness so that it yields incorrect results.
Interestingly, the order of brightnesses of the moons is not the order from largest to smallest moon. You might look up the sizes of these moons and see what that order is. What do you suppose would make a smaller moon brighter than another, larger moon?
Finally, another fact that is sometimes useful in determining which moon is which, are their orbital velocities. Moons that are closer to Jupiter move faster, and with shorter periods, than moons that are farther away. This may be very difficult to observe in some images, but you might try it out.