http://blog.csdn.net/lengshengren/article/details/25327481
cell.imageView.image = [UIImage imageNamed:[imageArray objectAtIndex:[indexPath row]]];
CGSize itemSize = CGSizeMake(60, 50);
UIGraphicsBeginImageContext(itemSize);
CGRect imageRect = CGRectMake(0.0, 0.0, itemSize.width, itemSize.height);
[cell.imageView.image drawInRect:imageRect];
cell.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
