Unfortunately not all the lovely images you see in your travels around the internet are available for you to use. Unless otherwise stated those images are copyrighted and belong to their creator. If you want to use them you will need to contact the creator and ask for permission and may have to pay a fee.
Fortunately there are images that you can use. These are either in the Public Domain or the creator has used a Creative Commons license so that they can share their work while retaining control of how it is used.
"Once the period of copyright protection expires, the work is in the ‘public domain’. This means that anyone can copy the work without having first to obtain permission from the copyright owner." Smartcopying - Students and Copyright
A Creative Commons license allows creators to share their works while allowing them to decide how their work is used. The video below give a great overview of how Creative Commons works.
OK, so now you should have an idea about how images are protected and when you can and can't use them in your assignments. If you have any doubts you should search for images that have Creative Commons licenses that will work for you.
Below are a bunch of wonderful websites where you can browse images for hours and hours...
Google Art Project is a wonderful platform where you can view high resolution images from wonderful galleries, museums, archives, libraries and other cultural instutions from around the globe.
The video below shows you briefly how to use the site.