Okay... Here's my explanation:
Image link is nothing more than displaying the Augmented Reality of the world around you onto your retina. The software behind it is a simple display interface and is as smart as your monitor's hardware on your computer.
Smartlink on goggles/glasses/contacts is an UPGRADE to an image link (you can't have Smartlink without it) and has a interface with your gun's Smartgun software to display a target reticule and the gun's information onto the lenses of the item. It uses Eye-Tracking software for you to interact with the gun's operation, making it so you look at and highlight the command "eject clip" to do things like that.
This is probably about as good of an explanation as I can hope for. The eye-tracking thing is a good way to go, although I wish they would have stated it directly somewhere. Thanks.
I think of it as a comparison between, say, MS Paint (image link) and a top of the line graphic art/image manipulation/whatever program (full on smartlink). In theory and in broad concept, both do the same thing. In practice, one is light years ahead of the other.
I might find this convincing if your smartlink provided a three-dimensional battle map with a ton of predictive information and AI-enhanced advice on what to do, like a TacNet. But it doesn't- it displays a targeting reticule, shows range and ammo count, and shows the expected trajectory for a target in motion. Not only is that not anything special in the year 2070, it's not anything special now, IRL. Your image link is probably accustomed to displaying lots more complicated data than your smartlink, with fully-animated ads and the like abounding in spam zones.
Final question: How do Smartlinks interact with riggers? If a rigger has a smartgun system on a vehicle he's jumped into, does he get the bonus automatically, not at all, or does he still need a smartlink (which would be weird, given that he's not using his own eyes...)?
Thanks for the replies, and happy hunting.