Can I use any image I find on Internet?
No, you can't.
At least not without certain restrictions. The author of an image has some rights on it, and you must not use it without the author's permission.
Which images could I use then?
Sometimes an image is copyright free. This means that the author permits people to use it freely. Even in these cases you will probably need to attribute it correctly, to show who the author is and recognize that work. These images are normally protected by CC licenses or Creative Commons Licenses.
You can find images under CC licenses here:
You can learn more about CC licenses here: (This link is in English, but there is probably a version in your language. Just search the Web)
What is "attribution"?
You make a correct attribution, when, together with the picture you use, you place the name of its creator and a link to the original image. In this way you recognize their work and make it clear that the image is not yours. You have an example of attribution of this image at the bottom of the page.
Is there any possibilty of using copyright free images without "attribution"?
Not normally. Even if you don't have to pay for using them or ask for permission, most creators require that you quote them properly and that you demand the same for those who use your image. However, there are some exceptions. This website, for example, only gathers images that are copyright free (also called "copyleft") and that don't require any attribution:
Thanks to Antonio Villanueva Echeverría who wrote this page.
Photo source: James Palinsad on flickr (Click here to see it) Used under CC LICENSE Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)