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					Originally Posted by  blutuu
					 
				 
				It works great now, but only for iBooks. 
			
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 That is because your others ereader doesn't support the property "background-size: contain;". In that case, and if we maintain the idea of images centered on the ereaders screen, we have to deal with the properties "display: table", "display: table-cell" and "vertical-align: middle"; but again those properties are going to be supported only by iPad and (Kf8) Kindles 
However if your images are always going to be of 594 x 768 then a more easy solution can be implemented. For example what you can see in this epub:
counting_with_pez_callor_brandon.epub
This ebook should work in any ereader