Being an author myself (unpublished at this point) I see the linear direction that I wish to take a reader of my books or short stories. They must follow me in a straight path to were I want them to go. They know nothing of my world until I am good and ready to tell them.
Words are the only weapons of the textual world. Is the night clear and cold? Warm and stormy? Is the character wet from the rain or his own sweat? The words are the only thing I currently use to make the reader understand what I want them to see.
Multimodal takes a different approach. A picture of the moon in the background over fresh fallen snow would tell the reader clear and cold. A picture of a man running through the rain would explain why he is wet.
Text added to the pictures would farther move the story. The clear and cold night could be a story of Christmas Eve. The man could be wet because he forgot his umbrella.
Perhaps adding a Christmas carol to the picture would stir the desired emotion of Christmas past. Dark, somber music may add a sense of foreboding the man running through the rain.
With today's inpatients with reading, the modes of communication quickly get information to the reader. More information can be sent to the reader before they loose interest. On-line reading is not like sitting by the fire with a blanket, fuzzy slippers and a good book. It is more like fast food. Get in, get out.
The on-line eye tracking research of Nelson (57) suggests that the reader scans the page in an "F" pattern. The eyes move across the top from left to right, through the middle then down the left. The book does not get into it that well, but it would suggest that the most important information should be on the top. Second most important in the middle, then links listed on the left that you consider important.
Unlike books, it is next to impossible to control how your reader gets their information on-line. They may read what you have written and look at your pictures or they may not. They might click on a link and never come back. Assuming the links are related, they may still find out what you want them to know.