This follows the increase in August of last year from 2 to 3.2 million pixels, and at long last brings the overall image size allowed more in line with the actual size of the device on which they are intended to be viewed.
The increase to 3.2 million from 2 million pixels was itself, of course, a long overdue attempt to address a contradiction in Apple's recommended image size as given in the iBooks Asset Guide. With a resolution of 2048 x 1536 since the 3rd generation iteration of the full-size iPad (giving it a total pixel count of 3,145,728), the long-standing limit of 2 million pixels was nowhere near enough to fill the screen with a full bleed image in portrait orientation, let alone to zoom in for greater detail, as the Asset Guide somewhat ironically recommends:
A quick bit of math will show that 1.5 times the standard iPad display resolution is 4,718,592 pixels. So while the new 4 million pixel limit does not provide for quite that much leeway, it gets us very nearly there, and at least allows for full bleed images that can be zoomed to some degree without completely losing fidelity. The official announcement for the newest update reads as follows:Apple recommends providing images that are at least 1.5 times the intended viewing size up to a maximum of 3.2 million pixels.
Increased Pixel Limit for Book ImagesAmazon, of course, has also recently increased the allowed image size in Kindle ebooks (see post here), although they use an overall image file size in megabytes rather than a pixel dimension limit. This allows for a much higher image resolution to fill the bigger 2560 x 1600 HDX 8.9 device, which boasts over 4 million pixels, before zooming (4,096,000 to be precise). By comparison, that would require an image containing 6,144,000 pixels in order to zoom to 1.5 times without interpolation! This is why the individual image limit in Kindle files has been increased to 5 MB. Converted mobi files for upload to KDP, however, are limited to an overall file size of 650 MB, which puts something of a practical limitation on the included image files (130 5Mb files to be precise, which is admittedly more than ample for most projects).
The pixel limit for all images within a book has been increased to 4 million pixels. This limit does not apply to images delivered separately from the book, such as cover art or screenshots.
Conversely, by employing a pixel limit, Apple are restricting the total image dimensions, but not the image quality itself, which can be embedded with no compression whatsoever, if so desired, up to a total ebook file size limit of 2 GB (which is a limit imposed by the zip format itself). So while the new 4 million pixel limit does not quite allow for full 1.5 times zooming with pixel-perfect accuracy, it does come very close, and the absence of a file size limit for individual images allows for the highest quality setting possible, which might be of great benefit for art books and photography collections (in case you want to see those brush strokes on the Mona Lisa, for example).
UPDATE: By the way, for those wanting to know, 2309 x 1732 would be the largest dimension image you can now insert into an iBooks file at the standard 4:3 aspect ratio of the iPad display. That gives you an image containing 3,999,188 pixels. Just one pixel bigger at 2310 x 1733 puts you over at 4,003,230 pixels. A nice round numbered 2300 x 1725 gives you an image with 3,967,500 pixels.