There is another way called rapid prototyping. In this, you would build directly into PowToon, but you would not get bogged down with details.
For images, you would put in shapes or textboxes describing ideas known as "placeholders". In many respects, it is the same as using a Word document or other template, or even a MindMap™ maybe.
The trick is not to get caught up in the details. Go through several iterations of thinking, getting closer and closer to the target. Think of it as a spiral going round and round, getting closer to the middle.
For a simple product to help with storyboarding, you might like to take a look at http://www.storyboardthat.com/.