This depends on which iPad you have.  

  

Retina iPads,  Air, Mini, 3rd and 4th generation iPads


The retina iPads need an image at least 2048×1536 pixels in size because that is the native resolution of the retina screen. 


Using a larger size may be worthwhile if you want to zoom into details or intend to output an image in some way such as emailing a high resolution image.  


There is no fixed upper limit on the size of the image you can add, but if you add massive images, i.e. : 22 megapixels or over then you will risk reduced performance for some operations. It won't negatively affect general viewing, but zooming in will be slower and less smooth if you load very large images. 


iPad 2


The iPad 2 needs an image 1024x768 in size.  


Using a larger size may be worthwhile if you want to zoom into details or intend to output an image in some way such as emailing a high resolution image.  


There is no fixed upper limit on the size of the image you can add, but if you add massive images, i.e. : 22 megapixels or over then you will risk reduced performance for some operations. It won't negatively affect general viewing, but zooming in will be slower and less smooth if you load very large images. 


iPad 1 


The iPad 1 needs an image at least 1024x768 in size.

Recommended maximum size 2048x1536 pixels.


Using a larger size may be worthwhile if you want to zoom into details or intend to output an image in some way such as emailing a high resolution image. However the RAM available in the iPad 1 is very limited and we'd recommend a maximum size of 2048x1536 pixels for this iPad.  


There is no fixed upper limit on the size of the image you can add, but if you add  images larger than the minimum you could experience crashes. 


iPad 1 was a breakthrough product in its time, however its long in the tooth now and realistically is not capable of delivering a professional presentation with current standards in digital photography now that even compact cameras and cellphones are capable of taking images over 12 megapixels. Foliobook will not be issuing any further updates for the iPad 1.