Cover picture on iPad album

Forgive me if this has already been covered before, but I can't find any. I have sync'ed an album of pictures from my iMac 10.6.8 to my iPad IOS 8.0. On the iMac I had a particuler picture as my first picture in the album. It came across to the ipad in the same position, first. But when I look at the album on the screen titled Albums, the picture shown as the cover picture is the 3rd picture in the album. On another album, it is the 4th picture.
How can I force a particular picture to be the album cover on the ipad?
Thanks for your help.
Kevin

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