思路：将UIImage转换成NSData，然后插入到NSMutableDictionary中，读取时，用NSData读出来，然后再转换成UIImage

－存储

UIImage *image = [self getPicture: id];
NSData *imageData = http://www.mamicode.com/UIImagePNGRepresentation(image);>
－读取
NSData *imageData = [object objectForKey:@"imageData"];
if (imageData) {
    UIImage *image = [[UIImage alloc] initWithData:imageData];
    imageView.image = image;
}
else{
    //...
}