The interface and back-end of the project is the container for the assets and content. In a typical web site the assets and content will take the form of ...
There are 4 primary ways to obtain assets ...
Including ...
Whatever bespoke assets are required which the client doesn't have. Quote for these.
See the various menus on this web site for image, video, audio etc content creation articles.
Ensure your brief is rock solid then seek quotes from suppliers (photographers, animators, copywriters etc). You can ask your client if they would like to attend photo shoots etc. Copyright issues are discussed here.
Do you need a pre-existing piece of video or audio? What are the copyright considerations.
However you obtain or create your assets, it is useful to write a project specification for file types, naming conventions, repurposing specs etc. More on asset logs here.