PageRank 4 steps

Why images are important aspect to take care when you do SEO for a website and how it helps to improve the traffic?
1. There is a huge traffic that comes to your site through images. Have images inbetween content for most optimized performance.
2. Image name: Do not keep whatever name you wanted or dynamic image names. [...]

Great, today the PageRank got updated and was reduced from PR5 to PR4. And all of my other websites went through the same.
Possible reasons could be:
1. The number of inbound links that did not increase after certain stage.
2. Sold one of my website and removed that link which had a PR6.
3. Added a new website domain with [...]