Any e-book that has an image needs an alt tag. What's that? A hidden-in-the-code piece of text that describes the art.
Say you use an image for a chapter opener (because you want the e-book to have the same typeface and design element as the print edition):
A reader who is using text-to-speech who comes across this image won't get any information other than "image." So, you need to add alt text, aka alternative text.
In this case, your alt text could read: "new chapter: 8, Nightshade Island."
Now, anyone using text-to-speech will hear that alt text.
What about a picture, say of the George Washington Bridge?
Your alt text could say "photo of george washington bridge." Or you could beef it up a bit and add "at night, with both towers illuminated." Give some detail.
If the image has a caption, make sure the alt text doesn't repeat the same information. If the caption to that bridge photo already labels it as a picture of the bridge at night, add some detail to the alt text, maybe "photo showing traffic backed up in both directions on the george washington bridge."
Alt text makes the publication accessible to everyone and provides a complete reading experience. If you're going to the trouble to design chapter openers or include images with important information, take the extra step to add the alt tag.
Here's a techy explanation of the hows and whys: