You can create images to use as Custom Slides in whatever image-editing program works best for you. This article gives some tips on how to create those slide images in PowerPoint.
Create Your Slide with the Right Dimensions
For custom slides in signage that does not use the sidebar, the slide image should be 1920 x 1080 pixels.
For custom slides in signage that does use the sidebar, the slide image should be 1440 x 1080 pixels.
At the bottom of this article, you can access Powerpoint files with those dimensions to help you get started.
Saving Your Slide Image in PowerPoint
Once you have created the look you want in your PowerPoint slide, you will need to save it as a .jpg image in order to upload it in the Share admin console. In PowerPoint, go to File, Save As, Save as Type, and then select JPEG File.
Once your slide image has been saved as a .jpg, you can add it to the admin console as a Custom Slide.