UITableViewCellに画像を設定するときなど。
CGSize size = CGSizeMake(100, 100);
UIImage *originalImage = [UIImage imageNamed:@"hoge.png"];
UIGraphicsBeginImageContext(size);
[originalImage drawInRect:CGRectMake(0, 0, size.width, size.height)];
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();