Just because you see the same Zac Efron photos on fansites and magazine sites repeatedly, it does not mean that you can download images of Zac Efron and download them for personal or commercial use. If you want legal pics of Zach Efron, you should use reliable sources of sites that have confirmed that the photographer is giving away free pictures of Zac Efron.
The pictures that you see of Zac Efron on Wikipedia are usually public domain photos that can be used as a Zac Efron backgrounds and wallpaper under a Creative Commons license. These photos are not of a high enough resolution to make Zac Efron posters, but a website can be decorated with pictures from the Wikipedia Zac Efron gallery.
Steps to using the photos To get these photos, look up Zachary Efron or Zac Efron on Wikpedia. You will see photos along the page that highlights Efron's biography of his personal life and career. Click on each photo that you want to use. If the license says that it is available, you will see the name of the photographer that you should use when you give credit to the photographer who took the photo.
Fanpop has a rule saying that you can not "copy or store any Content offered on the Fanpop Site for other than your own use." This means that you can use some of the Fanpop content for personal use only, and they encourage users to upload photos and share them with each other. You have to be careful though because sometimes people can upload photos that they think are available to share with everyone, but they are not.
Fanpop.com says "It is our policy to disable access to or remove material that we believe in good faith to be infringing on a copyrighted work." You can go to the Zach Efron section of Fanpop and use photos of Vanesa and Zac Efron or just Zac for personal use if the site allows that the photo is copyright free.
In addition You can also find free Zac Efron avatars and images from the projects with which he associated. For example, Disney offers High School Musical pictures and graphics for fans to post on their Facebook pages and use on websites when they are promoting shows.