C process .bmp file

Hi, I am supposed to read in a .bmp file and then alter it based on command line arguements.

Examples:
-fromrow x, where x specifies the bottommost row to process
-torow x, where x specifies the topmost row to process
-fromcol x, where x specifies the leftmost column to process
-tocol x, where x specifies the rightmost column to process
-op x, where x is one of the following
-- 1= threshold the image(any pixel value in the specifies range over 127 is
changed to 255, and pixel values 127 or less is changed to 0)
-- 2= negative (any pixel value p in the specified range is changed to 255-p)

I have been given this code as am example of reading a .bmp file:

Code:

---

#include <stdio.h>
#include <stdlib.h>
int main(){
FILE *fp= fopen("sample.bmp", "r+");
if (fp == NULL){
printf("Error");
}
int temp=0;
//Go to Byte 22
fseek(fp,22,SEEK_SET);
//Read byte 22 into an integer variable
fread(&temp, sizeof(int), 1, fp);
printf("Number of Rows: %d\n", temp);

fseek(fp,18,SEEK_SET);
fread(&temp, sizeof(int), 1, fp);
printf("Number of Columns: %d\n", temp);

fseek(fp,10,SEEK_SET);
fread(&temp, sizeof(int), 1, fp);
printf("Start of Pixels: %d\n", temp);

fclose (fp);
}

---

I don't even know where to start in terms of altering an image... :/
ANY help/advice/linfo/links would be greatly appreciated.

Thank you in advance.