We reported a few days ago a fix for users that were having problems with iBooks on iOS 5 jailbroken device. Since then the Corona 5.0.1 untether package in Cydia has been updated to fix the problem. Updating or installing this package is far easier than re jailbreaking using redsn0w, so we recommend this alternative. If this is not supported by your device use this alternative.
If you are having problems with iBooks follow these steps.
1. Open Cydia and search “Corona 5.0.1 Untether”
2. Install the package and restart your device if requested
3. iBooks will now load images
Corona 1.0-6: @planetbeing put a lot of work into solving iBooks image loading bugs. All users (however jailbroken) on 5.0.1 should install.
— Jay Freeman (saurik) (@saurik) January 31, 2012