this may help you insert image into rich text
public void InsertImage(string pic)
{
string lstrFile = pic;
Bitmap myBitmap = new Bitmap(lstrFile);
Clipboard.SetDataObject(myBitmap);
DataFormats.Format myFormat = DataFormats.GetFormat (DataFormats.Bitmap);
if(richTextBox1.CanPaste(myFormat))
{
richTextBox1.Paste(myFormat);
}
else
{
MessageBox.Show("The data format that you attempted site" +
" is not supportedby this control.");
}
}