At now Custom objects don't contain binary data itself. As a possible variant you can do the next:
- load your image as a BLOB (http://quickblox.com/developers/SimpleSample-content-ios). After upload you get URL to uploaded file.
- save this url to custom object as a string.
QuickBlox team is going to release iOS SDK v.1.8 with CustomObjects file API soon, feel free to check developers section http://quickblox.com/developers/IOS#Download_iOS_SDK