Use built-in slide layouts!
Add alternative text for all meaningful images, charts, graphs, and objects.
For guidance on how to do this on your own, check out:
Creating Accessible PowerPoint Presentations
Video: Improving image accessibility in PPT (1:38)*
For multiple images on one slide, group image and then add alt text for the grouped image.
For guidance on how to do this on your own, check out:
Group or ungroup images, shapes, or other objects in Office*