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.
Here we have listed some image collections accessible via the Internet where you can access Creative Commons (CC) or Public Domain images to use in your assignments. Creative Commons materials are resources made available by their creators to be used in a particular way under certain circumstances. If using a Creative Common (CC) image you are required to attribute or make a reference in your assignment.
It is good practice to attribute all images used in your assignments. Below is an example of an Creative Common Flickr image attribution.
Google Art Project is a wonderful platform where you can view high resolution images from wonderful galleries, museums, archives, libraries and other cultural institutions from around the globe.