The alt image carries no weight for normal SEo on page optimisation. It IS best practice to use it due to usability, but IF the image is a link, then the image tqakes on new importance as seaerch engines place more weight on links. With the image as a link, the algorithm has no knowledge of what the image is about, so the text used in the ATL atribute becomes the equivalent of anchor text.
But, as matt C says, if you spamm your alt attr, then you will be seen as a spammer, in much the same way athat the keywords meta is ignored unless it is abused.
As for the 'bridge', it is simple. There are three main elements in a link
1. anchor page
2. target page
3. anchor text
The anchor text is a bridge between the two pages, by this I mean to say that it is seen by the algorithm as appearing on BOTH the anchor page AND the target page. It is effectively lifted up, and placed on the anchor page (which is why the anchor text is so important). Now there are alos huge factors like semantic relevance, and if when the anchor text is placed on the target page it has semantic relations to the content, then again it becomes a far more powerful link.
Hopefully that has explained the bridge function of the anchor text.
