Image link doesn't take up capacity. I think the explanation is that it's so common that everything has the hardware in order to be compatible, while others like low-light vision and ultrasound might need additional components.
Where does it say that Image Link doesn't take up capacity?
It explicitly states in Ultrasound that it takes up 2 Capacity, but doesn't have any statement in Image Link that would otherwise counter the statement that each enhancement takes up 1 Capacity. The fact that Image Link is extremely common is represented by the fact that it doesn't add to the vision enhancer's availability, and is also very cheap.
I've always viewed Capacity on Vision Enhancers (and Audio) to be its ability to run various inate "software", Image link is still an enhancement that the glasses would need to run, so it still takes up capacity. It is of course possible to have glasses without the Image Link.
Image Link is included and takes no Capacity in Cybereyes and many helmets, but not Vision Enhancers (glasses, goggles, etc.). It is an enhancement and therefore takes up capacity. In fact on page 44 of Arsenal it lists Image Link as taking a Capacity of [2], though that is specifically in relation to armor.
Barring anything to the contrary I would say RAW is that Image Link takes [1] Capacity in a Vision Enhancer device.
Also, Capacity is hardware requirements, not software. That's why a Radar takes [5], and Thermometric takes [1]. Image Link is just video display, if it was software it would barely take any software capacity, even on a device today. It's the hardware that is the big limiter.