Be careful when you use material from the Net or from other people. As your teachers mentioned you, you must always quote your sources. And in many cases this material might be subject to authorship rights and CANNOT BE USED WITHOUT PERMISSION.

Always use material made by you or your partners. Or be sure that it is copyright free, that's to say, under Creative Commons Licenses or similar. Have a look at this page from another project to refresh the main ideas:

Using Images, Music,... from the Net

Have a look here to learn about Creative Commons (Source = Wikipedia)