Please find this link, this might be very useful
csexrichtextbox
And if you want to read images from Project Resource then you can use the code sample below;
System.Drawing.Image tempImage = MyProject.Properties.Resources.myImage;
Clipboard.SetDataObject(tempImage);
DataFormats.Format image = DataFormats.GetFormat(DataFormats.Bitmap);
richTextBox1.Paste(image);