You start always with existing images. These may either be images you import or images you create with the generators. All these nodes are in the Input menu.
You can import single image sequences with the Image Sequence node. The Image Sequence node supports all file types known by QuickTime. Click on the Folder parameter to locate the images. The node imports all images of the folder, ordered alphabetically.
You can import a QuickTime movie. The node will import each frame of the movie.
You can import PDF files. You define the size of the rendered files. Each page gives a frame.
You also have generators like Color Bars and Solid.
The Current node is a special input node. It uses the output of the current active node.
You can also import text files. Textfiles combined with Graphic nodes can be used to create serial titles like subtitles or lower thirds.
For all input nodes, you will also have to define the aspect ratio. The correct aspect ratio allows for corrrect display of non-square pixels of the video formats and also allows for aspect ratio correction of some of the nodes.