Here's couple of annoying requirements from the newest edition of iBookStore Asset Guide 5.1.
For accessibility , the alt attribute for an image must be included; the value must be an appropriate replacement for the image. It should just be what you would have put in the prose if the image was not included , as shown in the following example:
<p>The hillside was covered in poppies.
<img src="images/page1/flowers.jpeg" alt="The poppies are red, orange, and
yellow, and a winding path leads to a house.">
A dog was asleep on the porch.</p>
The following example is not as effective; the alt text only describes the image, instead of being a textual replacement for the image. When reading without the image, the text does not flow as well as the example above.
<p>The hillside was covered in poppies. <img src="images/page1/flowers.jpeg"
alt="A bunch of poppies and a house."> A dog was asleep on the porch.</p>
The following alt attributes are not acceptable: alt="none", alt="nothing", alt="image", or
alt="page 3". Leaving out the alt attribute is also not acceptable. The attribute alt="" is acceptable in cases where the image is decorative and does not have any content or meaning
Personally I wish this was not necessary for iBooks publishing. I mean, if you make a fashion type eBook with lots of images. Visually impaired readers who need alt tags are not even consumers of this type of eBook.
It's just a pointless time consuming addition to the production flow for some type of eBooks.