I've been reading for some time on those legal issues but I can't understand somethings: If in my site I'm writing an article about for example David Beckham, can't I use a small picture of him? even If it's an ordinary picture published every where on the internet? and how about linking to some of his goals on you tube? If a site is about software and it has some print screens of software, is this illegal? If those actions are illegal then why millions and millions of sites, blogs...etc are doing this?!
You are right what I do is always give credit to the original author, use site like flickr that way you know who the author is and you can easily give him/her a credit. before that though you need to be sure the you have the right by CC to republish the picture.
source: http://www.copyright.gov/title17/92chap1.html#107 Does this mean my examples in the above questions are ok?
Is your site commercial i.e. generates any type of revenue (shows ads, donations for bandwidth, memberships)? Generally speaking, fair-use doesn't apply to 99% of websites and there are a lot of sites that have been sued for trying to claim fair use. You should get permission for any image you use. Yes, there are a lot of sites that are guilty of copyright infringement - but it isn't a legal defense and you are taking a risk of being sued for a usage fee by the photographer or copyright holder - and usage fees of several thousand per image is not unusual. Copyright infringement can also be a criminal act and there are cases of extreme violators going to prison for several years.
Almost all of the free images sites I've seen demand that you mention the photographer name and a link of the website, does it mean I have to write those under every image I use, even small thumbnails? If so then it would be really very bad. Also what does free for non commercial use mean for images? If you use those kind of images in your site and your site has some ads, does this mean you are violating the license?
Yes, you would need to do it with every photograph that requires it. Another way to look at is- why should they go to the time, expense and trouble to take a picture so you could use it for free? Take your own picture if you don't want restrictions. "Commercial" generally means that the site takes in revenue and isn't a legal non-profit entity. Ask them specifically if you want to know their exact terms.
I wouldn't say so. There are loads of people on Flickr who upload images from other sites/images they haven't created and pretend that they created them. They also publish these under the CC, which is totally wrong. For the most part I would suggest that you stay away from them.
I don't think you need to credit the photographer for small thumbnails. A case recently lost against Google because they claimed that Google used their pictures without permission, but it was ok because it was thumbnails.