sample book pics in ibooks store are blurry

I just uploaded a children's book from ibook author to the ibook store. Everything look great except when viewing the sample pages in the ibook's store, they are blurry. The images I used for those screenshots have the following resolution: 2048px x 1496 with a resolution of 600 px per inch. (They were saved as a jpgs from photoshop. Should these images be saved at a different size? I couldn't find an reference to the screenshots that are used for the sample pages in any of my searches online.

Thank you