Activelink on Freenet is a small image that is inserted together with the freesite to signify the existence of that site. On Freenet 0.7 it has become customary to place an activelink within the site's container, which means that if you are able to see the activelink of the site, then you have already downloaded the entire site.
Activelinks are used to give a link to another site in a visual manner, to create a list of subscriptions while speeding up the delivery of sites, etc.
However, the use of activelinks has received much criticism. For example, opponents will say that the page that links to sites via activelinks will itself load significantly slower, and potentially cause the browser to stop downloading other pages, which an activelink is being loaded.
In earlier version of Freenet (0.5) the activelinks were of different formats, but in 0.7 only PNG are used. This means that the name of the activelink has become predictable, and if site provides an activelink it will be available in /activelink.png
The dimentions of activelinks are not a standard, but a loose convention. Most sites will use 108x36, but other dimentions have also been used by some. 108x36 is useful due to the fact that it can be scaled by 2, 3, 4, and 6.
In Freenet 0.5 imagelink dimentions were 95x32 and not limited to a single format, many people used GIF (especially when wanting a small animation).
Activelink also should have never reside within the site's container.