There seems to be lots of confusion about image sizes and Apple iBookstore Support et al simply state that images should not be greater than 2 megapixels.
IBook Author Help > Photos and other images, states:-
Important: When you export your book for distribution outside the iBookstore, images are automatically scaled to a width of 2048 pixels, which is optimized for iPad with Retina display. Readers can view these images on other iPad models, but the viewing experience might be slower. You can improve the viewing experience on iPad models without Retina display by using images no wider than 1024 pixels.
Publishing to the iBookstore:
If most of your readers will be using the original iPad, scale down images in Interactive Image widgets to a width of 2048 pixels and all other images to a width of 1024 pixels.
Previewing on iPad, or exporting for distribution outside the iBookstore:
You can improve the viewing experience on iPad models without Retina display by scaling down images in Interactive Image widgets to a width of 2048 pixels and all other images to a width of 1024 pixels.iBA help deals with exporting books OUTSIDE the store and simply advises that you can help none Retina viewers by having a smaller image. No help at all really - but suggests that the use of images 2048 is OK. that would indicate 2048 x 1536 or 1496. No mention of what resolution should be used.
iBookstore support basically reapeat this.
So to ease life in getting books accepted, should we all use 1024 x 768 @72ppi for iPad & iPad 2 and now iPad mini. ????
and use 2048 x 1536 @ ?? for The New iPad Retina display!
Both fail to mention HOW the books author would know which iPad a buyer/viewer will be using. Also if "we" only use 1024 x 768, those with Retina do not get an "enhanced viewing experience".
For a company that deals entirely with devices that use logical programming - there seems very little logic applied when it comes to dealing with a section of their business which by issueing iBA Free and wacking 30% of the top, must be adding to the $billions already stacked away in a corner somewhere.
iBA makes eBooks for use/display on iPads only. For months now since iBA became available, one of the most common requests on this forum is help with....image sizes.
It seems reasonable now when Retina is in use and eventually the iPad and iPad 2 will fade away - ditto the new none retina mini and ALL Apple devices and computers will be retina. So why not plan now! Surely images in shoudl be geared to deal with Retina displays. IF Apple would take that line maybe "Support" would have less work dealing with requests for help about images IF Apple accept that the next Stephen King may not have a clue how to deal with adjusting images sizes and simply state the required dimentions.
I make photo books and use 2048 x 1496 @150ppi - so far ten free books have gone into the store.
The suggestion about images for retina display as "standard" as been sent as feedback.