Just as a note on this as well. Anyone can be an affiliate, there are no limitations on it. The best affiliates are 'content' affiliates who offer original content such as a blog and link to external websites as part of it. So with allposters you wouldn't be able to use their content for free to advertise their products, but if you wrote an article about images and where to find them, you can link to allposters and receive commission for the traffic/sales generated.
There are more than Google Adwords, remember that people. There are affiliate networks where the retailers directly offer commission for sales. In American look for Linkshare, Digital Window, Webgains, ShareASale. They will approve you onto programs, you get a link and you just apply it to the website. Every sale you generate tracks in their system and commission is paid to you if the sale isn't cancelled. In the UK you can find them at Affiliate Window, Tradedoubler, Linkshare, Webgains and more but start with those four.
