In SharePoint you'll use libraries to store documents and images. There are several different kinds of libraries, but on our public-facing websites (the ones that start with www) we only usually use the document, picture, and pages libraries.
The document library is where you should store things like Word documents, PDF files, Excel files, and Powerpoint presentations. Every site is created with one document library by default, but you can create more for organizational purposes if you'd like.
No picture libraries are created by default with the site, we create them as we need them. Usually each picture library is created to hold images used for a specific purpose. For example we'll create a picture library to hold the images for a slideshow or to hold all the faculty photos for a faculty list page. In those cases rather than specifying which individual images to use in a slideshow the slideshow will be configured to use all of the images present in a specified picture library. This makes maintaining the slideshow easier since you don't have to edit the code when adding a new image, you just need to upload the image to the correct library.
Each site also has a single pages library, as the name implies this is where all of the pages for your site are stored.
Customizing the Libraries
You can customize libraries in the same ways that you can customize lists, you can add columns to store additional information along with the document, and you can create different views for looking at sets of documents. In a sense libraries are really just specialized lists of documents, instead of lists of small bits of data.
You can also embed a view of a library directly into your page. Below is the document library for this website.