It's not complicated, you're just not doing things right. There area few ways you can fix this.
The first is by keeping a list of points that the user has drawn, then when you go to save loop though that list and draw each of those points to your bitmap (the one that you get from the panel1.DrawToBitmap() method). That's a better choice.
The second option is create a global Bitmap and Graphics object, call their constructors in the Form constructor (Form1()). Construct the bitmap to be the same width and height of the panel. set panel1's backgroundimage to the bitmap. Finally, Set the graphics object to Graphics.fromImage(panel1.BackgroundImage).
Now, in your mouse move event remove the Graphics g = panel1.CreateGraphics() and g.Dispose() calls (that's where you were going wrong), just do the g.Fillllipse(), then add this line:
panel1.Invalidate(new Rectangle(e.x, e.y, 10, 10));
Which will force the panel to repaint itself when you draw a point.
Finally in your loadToolStripMenuIem_Click event handler, comment out the panel1.BackgroundImage = Im line and add this one:
g.DrawImage(Im, 0, 0, panel1.Width, panel1.Height);
Basically what you're doing is creating a bitmap the same size as the panel, setting it as the background image, then creating a graphics object out of that image. Using any of the Graphics methods will draw to the bitmap (which is the panel's background image). You're literally drawing directly onto the background image. In your save event comment ut everything in the try block and do this:
panel1.BackgroundImage.Save(save.FileName);
That's it. Now, keep in mind, this is a hacked together solution so that I wouldn't have to modify too much of your existing code. There are MUCH better (more efficient) ways of doing this (like the first option I gave you). I really recommend you take another look at my application, turn on the file list option and watch what happens as you draw. The source code for that program is available, but I don't want to give it to you just yet. Play around with what I've taught you here and see if you can learn anything.
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