I found this out by accident when i was copying and pasting parts of my site to different pages. Each page had 5 images, being lazy in the beginning i had left one of the images alt text as lets say "Bass Fishing" The site had about 40 pages. I was very surprised to find that my rankings for "Bass Fishing" were very high considering the site was new, about 4 months and i had only done mild Off page seo. So the moral of the story is if you have a keyword that is really hard to get it may help to insert an image onto other pages with this alt text as i believe that Google will give that site as a whole more relevance to that keyword. This could also be a variation of Siloing a site. A theory im experimenting with. Try it out, go through some of your pages and add in an image (doesn't have to be huge, but not tiny either) and see what happens. Please post anything you've learned... Thanks!