WordPress keeps your pictures and files in a separate area called the Media Library. This is a little different from some website design systems, but it does allow you to see what files you already have, and you can use the same image on different pages without having to upload a new copy.
Easy WP Guide gives a good overview of the media library listing page, along with further pages which show how to add new media items to your library.
Replacing an existing item
If you store data files such as PDF files, you may at some point wish to update the file, for example if your constitution or membership form is revised.
You can do this from the media library using the Replace Media link which you can see in the list view when you hover the mouse over the item, or in the grid view when you click on the item. This allows you to upload a new file which will completely replace the old one.