关于点击按钮图片高亮显示

我想要在点击按钮时，设置默认图片高亮显示。按钮代码如下，在执行这段代码时，怎么设置另一个UIImage高亮？

- (IBAction)buttonOperationPressed:(id)sender {
    if (currentOperation == 0) result = currentNumber;
    else {
        switch (currentOperation) {
            case 1:
                result = result + currentNumber;
                break;
            case 2:
                result = result - currentNumber;
                break;
            case 3:
                result = result * currentNumber;
                break;
            case 4:
                result = result / currentNumber;
                break;
            case 5:
                currentOperation = 0;
                break;
        }
    }
        currentNumber = 0;
        calculatorScreen.text = [NSString stringWithFormat:@"%g", result];
        if ([sender tag] ==0) result=0;
        currentOperation = [sender tag];
        userInTheMiddleOfEnteringDecimal = NO;
  }