12. A non-transitory machine-readable medium storing a program for generating a low-resolution version of an electronic document at a content distribution system, the program executable by at least one processing unit, the program comprising sets of instructions for: from a content publishing application, receiving (i) a high-resolution version of the electronic document that comprises a plurality of high-resolution images and (ii) a markup language file associated with the high-resolution version of the electronic document, the markup language file comprising a plurality of annotations that each specify a resolution for a low-resolution version of a respective high-resolution image, the specified resolution identifying a maximum pixel width and pixel height resolution at which the respective image will be displayed on client devices having a particular resolution, wherein the low-resolution version of the electronic document is not received by the content distribution system from the content publishing application; creating, at the content distribution system, the low-resolution version of each respective high-resolution image of the plurality of high-resolution images based on the respective annotation of the plurality of annotations; generating, at the content distribution system, the low-resolution version of the electronic document using the created low-resolution image; and storing (i) the high-resolution version of the electronic document with the high-resolution image for distribution to a first plurality of client devices and; (ii) the low-resolution version of the electronic document with the low-resolution image for distribution to a second plurality of client devices having the particular resolution.
15. The non-transitory machine-readable medium of claim 12 , wherein the electronic document is one of an electronic book, an electronic magazine, and an electronic newspaper.