Dragging RichTextBox, Button from Visual Studio toolbox to your form designer.

Adding a click event handler to the Open button allows you to select an image, then load image in RichTextBox control in C#.
private void btnOpen_Click(object sender, EventArgs e)
{
using (OpenFileDialog ofd = new OpenFileDialog())
{
ofd.Filter = "Image Files(*.jpg; *.jpeg; *.gif; *.png; *.bmp)|*.jpg; *.jpeg; *.gif; *.png; *.bmp";
if (ofd.ShowDialog() == DialogResult.OK)
{
Clipboard.SetImage(Image.FromFile(ofd.FileName));
richTextBox.Paste();
}
}
}
We will use OpenFileDialog class to open a dialog allows you to select an image, then copy the image to Clipboard.
Finally, you need to paste to RichTextBox control.