This article goes over how to upload an image from within the slide editor. This is useful when you want to change an image with a new one that you have saved on your computer but has not been uploaded to the Image Library yet.
How to Upload an Image From the Slide Editor:
Navigate to the Image Library via
Edit Mode or 
Layout Mode.
Click the
Upload Images in the top left of the library.
Navigate your computer and select the image(s) you want uploaded to the Image Library.