imshow() sometimes don't clear previous image

edit

 #include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <stdio.h>
#include <string>
using namespace cv;
using namespace std;

int main()
{
    String folder = "F:/8TH SEMESTER/Traffic Sign Dectection/30speed/*.ppm";
    //string folder = ("F:/Hammad Profile/FYP/German/GTSDB/FullIJCNN2013/*.ppm");
    vector<String> filenames;

    glob(folder, filenames);

    Mat myImage;

    for (size_t i = 0; i < filenames.size(); i++)
    {
        myImage = imread(filenames[i]);
        imshow("Sample Traffic signs", myImage);
        int c = cv::waitKey(400);
    }

    return 0;
}

"For example if 1st image is display and then 2nd so it display it but some portion of 1st image is show. why this happen.