After about 4 years selling vectors files on microstock sites I will create my own blog (nothing related with vectors or microstock) and i will also buy some stock pics to use as visual resorces for my articles. Honestly i never understand anything about licenses, i just sell my images on sites, and i understand that when i make a big sale it is because somebody buy a "special licence" but never take my time to understand it.

Now i am in the other side, i will buy some images to use in my website, so i need to understand licenses; my big question is, i need a special license to use an image on my website? or i can just pay the minimum for the image to use in this way?

That would be a normal license for any of the sites I can think of. The only thing to watch is that some licenses restrict the resolution you can post an image on the web.