Problem to writing to file in C

Take a look at this solution in C++, looks like the same you want to do. You can adapt it to C-Language:

parsing - How to read in data from a pgm file in C++ - Stack Overflow[^]

#include <iostream> // cout, cerr
#include <fstream> // ifstream
#include <sstream> // stringstream
using namespace std;

int main() {
  int row = 0, col = 0, numrows = 0, numcols = 0;
  ifstream infile("file.pgm");
  stringstream ss;
  string inputLine = "";

  // First line : version
  getline(infile,inputLine);
  if(inputLine.compare("P2") != 0) cerr << "Version error" << endl;
  else cout << "Version : " << inputLine << endl;

  // Second line : comment
  getline(infile,inputLine);
  cout << "Comment : " << inputLine << endl;

  // Continue with a stringstream
  ss << infile.rdbuf();
  // Third line : size
  ss >> numcols >> numrows;
  cout << numcols << " columns and " << numrows << " rows" << endl;

  int array[numrows][numcols];

  // Following lines : data
  for(row = 0; row < numrows; ++row)
    for (col = 0; col < numcols; ++col) ss >> array[row][col];

  // Now print the array to see the result
  for(row = 0; row < numrows; ++row) {
    for(col = 0; col < numcols; ++col) {
      cout << array[row][col] << " ";
    }
    cout << endl;
  }
  infile.close();
}

One reason could be that the characters are in the image file itself :)

ÍÍÍ characters are assigned to uninitialized variables in debug build.
How have you defined buffer? If you defined buffer as an int (or short), you will get these characters in debug build because:
i) you are reading an array of bytes in buffer.
ii) you are writing every 4th byte (from buffer) in file.

It should work if you change buffer to char.