Of course it does - the StreamWriter creates a new file each time you call it!
Use the File.AppendText method:
using (StreamWriter sw = File.AppendText("xk24.txt"))
{
sw.WriteLine(imgcoor[0].ToString() + "," + imgcoor[1].ToString() + "," + imgcoor[2].ToString() + "," + imgcoor[3].ToString());
}
Plus, it is a VERY good idea to encapsulate your file access in a using block - until the file is disposed, it may not be available for more writing.