Answer:
You can use a drawing software. It usually included without having to download the new one.
Step-by-step explanation:
Without preparation, you wouldn't have enough time to actually make a good graph or selective image to aid you with the presentation.
One way to handle this is by extending your monitor to a projector and use a drawing program.
The drawing program provide you with the ability to create simple shapes and free-pen tools. So, as you explain your material to the audience, you can draw assisting image to help get your point across. This will be similar to a whieboard that your teacher use to explain material in high school.
That's being said, this method wouldn't be as effective compared to actually making preparation beforehand.