For testing I used this user created ballpark pack from OOTP:
http://www.ootpdevelopments.com/boar...dium-pack.html
Then I changed the size to fit the needed size for the iPhone/iPad:
iPhone: 480 * 256
iPad: 1024 * 640
Then name it exactly like the ballpark, e.g.:
iPhone: Fenway Park.jpg
iPad: ipad_Fenway Park.jpg
Last step is adding them to the game via iTunes Share.
To test you can use my exampes:
iPhone/iPod
iPad
Of course you can use every pic you want so look around this side or search the internet. I'm sure in a few days a great pack will show up.