Alpha blend watermark bmp in image bmp in c++

I have a task at school to add watermark bmp image into some other bmp image. The task is called alpha blending. I have to insert watermark at specific coordinates which user will set through program parameters on start, as well as alpha value for watermark blending. I am almost succeed, but I am getting small error. Here is the code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BMP_SIGNATURE_0         0x42
#define BMP_SIGNATURE_1         0x4D
#define BMP_DEPTH               24
#define BMP_HDR_SIZE            24

int isValidBmp(unsigned char *header)
{
    if (header == NULL)
    return -1;

    if ((header[0] == BMP_SIGNATURE_0) && (header[1] == BMP_SIGNATURE_1))
    {
        if (header[28] != BMP_DEPTH)
            return -1;
        else
            return 0;
    }
    else
        return -1;
}

unsigned long getBmpWidth(unsigned char *header)
{
    unsigned long width;

    if (header == NULL)
        return 0;

    width = ((unsigned long)header[21] << 24) |
        ((unsigned long)header[20] << 16) |
        ((unsigned long)header[19] << 8) |
        (unsigned long)header[18];

    return width;
}

unsigned long getBmpHeight(unsigned char *header)
{
    unsigned long height;

    if (header == NULL)
        return 0;

    height = ((unsigned long)header[25] << 24) |
        ((unsigned long)header[24] << 16) |
        ((unsigned long)header[23] << 8) |
        (unsigned long)header[22];

    return height;
}

int main(int argc, char *argv[])    
{
    FILE *fIn, *fOut, *fWaterMark;
    unsigned char *mIn, *mOut, *mWaterMark;
    unsigned long fsize, zsize;
    unsigned long fwidth, fheight, zwidth, zheight;;

    fIn = fopen("D:\\Downloads\\image.bmp", "rb");
    if (fIn == NULL)
    {
        printf("ERROR!\n\n");
        return 1;
    }

    fseek(fIn, 0, SEEK_END);
    fsize = ftell(fIn);

    mIn = (unsigned char *)malloc(fsize);
    fseek(fIn, 0, SEEK_SET);
    fread(mIn, sizeof(unsigned char), fsize, fIn);
    fclose(fIn);

    if (isValidBmp(mIn) == -1)
    {
        printf("ERROR!\n\n");
        free(mIn);
        return 1;
    }

    fwidth = getBmpWidth(mIn);
    fheight = getBmpHeight(mIn);

    if ((fwidth == 0) || (fheight == 0))
    {
        printf("ERROR!\n\n");
        free(mIn);
        return 1;
    }

    fWaterMark = fopen("D:\\Downloads\\watermark.bmp", "rb");
    if (fWaterMark == NULL)
    {
        free(mIn);
        printf("ERROR!\n\n");
        return 1;
    }

    fseek(fWaterMark, 0, SEEK_END);
    zsize = ftell(fWaterMark);

    mWaterMark = (unsigned char *)malloc(zsize);
    fseek(fWaterMark, 0, SEEK_SET);
    fread(mWaterMark, sizeof(unsigned char), zsize, fWaterMark);
    fclose(fWaterMark);

    if (isValidBmp(mWaterMark) == -1)
    {
        printf("ERROR!\n\n");
        free(mIn);
        free(mWaterMark);
        return 1;
    }

    zwidth = getBmpWidth(mWaterMark);
    zheight = getBmpHeight(mWaterMark);

    if ((zwidth == 0) || (zheight == 0))
    {
        printf("ERROR!\n\n");
        free(mIn);
        free(mWaterMark);
        return 1;
    }

    fOut = fopen("D:\\Downloads\\new_image.bmp", "wb");
    if (fOut == NULL)
    {
        free(mIn);
        free(mWaterMark);
        printf("ERROR!\n\n");
        return 1;
    }

    mOut = (unsigned char *)malloc(fsize);
    fseek(fOut, 0, SEEK_SET);

    double alpha = 0.5;    

    memcpy(mOut, mIn, fsize);
    unsigned int index = BMP_HDR_SIZE;
    unsigned int x = 200, y = 200;
    for (unsigned int i = BMP_HDR_SIZE + x*y; i < x*y + zsize; i++)
    {
        unsigned char v = ((1 - alpha) * mIn[i]) + mWaterMark[index++];
        mOut[i] = v;
    }
    fwrite(mOut, sizeof(unsigned char), fsize, fOut);

    free(mIn);
    free(mOut);
    fclose(fOut);

    return 0;
}

Thank you.