The problem is that you are drawing directly on the image:
Graphics offScreenDC = Graphics.FromImage(pictureBox1.Image);
Which means that the Image held in the PictureBox control is modified directly. The only way to "undo" those changes is to reload the image from the original source.
BTW: If you construct a Graphics object as above, then you are responsible for calling Dispose on it - if you don't, then your program will crash unpredictably, as Graphics contexts are scarce resource.
Personally, I would handle the PictureBox Paint event, and draw all the required lines there using the Graphics context supplied, rather than drawing on the Image directly (unless you intend to save the Image afterwards, and even then I would just apply the changes immediately before I wrote the file).