After you create the structure of your document, go back and add the content. The main reason for creating empty topics first is so that you can create links and cross-references to them while writing other topics. While you're writing, you'll always find a few topics that you haven't anticipated, and it's simple to add these. Creating the structure at the same time as the content distracts you from completing either task well.
As you are writing and assembling your content, you'll create File objects for each graphic, and put them in the topic.
There will probably be several rounds of changes during the rest of the authoring process - not only to the text but also to the graphics. You may need different formats for your graphics for each output. If you're handling graphic format conversion yourself, you'll only want to do it once, after things are finalized.
We recommend publishing the content to your chosen output formats at least once, so you can see how well your information structure and format are working.