Careful cizko!
Free doesn't automatically mean you can violate their copyright, should they maintain that. Often, those "free" things are only free under special conditions, e.g. you may use them privately only, but once you use/show them publically on the web, you violate that.
And if you, as an apparent German reader, read the c't Magazin regularly, you should have noticed that there are actually companies that make their money thru this: They suggest to people they can use their free pictures, only to send them a big fee once these pics appear in a Myspace account as a background picture or something like that.