Alternative Text is a description of an image within the context of the content on the page.
More information about alternative text
If decorative images do not provide additional content to the information on the page then a null alt tag is sufficient. This would be an empty alternative field in WordPress.
Complex Images & Long Descriptions
Complex images tend to be images of charts and diagrams. Complex images have both an alternative text and a long description of the image. The long description is either on another webpage or at the bottom of the current webpage.
More information about complex images and long descriptions
Images with Text
If the same visual presentation can be made of the text alone then an image may not be used to present the text.