c# transparent image overlay

Don't do a big common mistake: don't do your on screen rendering using PictureBox!
As much of your work is dynamic rendering of the images on screen, I would avoid using PictureBox at all. (You could use it, but I cannot see where it helps in your case except in eating up extra resources and performance.)

So, use some Control as a container for graphics. It can be some Panel, but it can be more convenient to create your custom control class derived from <code>System.Windows.Forms.Control. Do all rendering in overridden OnPaint method of control. If you don't want to create a custom control, use Panel.Paint event handler. In both cases, use the instance of System.Drawing.Graphics you get from event arguments parameter (never create Graphics instance using its constructor).

You should do all changes of graphic using some data model. The rendering event only translates the data model by rendering it on the Windows message WM_PAINT. For example, your data includes an instance of Bitmap (your photo), its position, positions and parameters of some graphics primitives you put on top — texts, rectangles, etc.: colors (with alpha channel showing opacity), sizes, orientation. Draw your photo at the very background using Graphics.DrawImage. (You see, you don't need PictureBox.) You can support some container of graphical elements.

Now, when you drag an element or change some parameters, you need to refresh the image. The only valid way to do it is calling Control.Invalidate. Use Invalidate with parameters (Rectangle or Region) to improve performance: these method will invalidate only a part of the scene.

In your control, don't forget to use Control.SetStyle to add the style System.Windows.Forms.ControlStyles.OptimizedDoubleBuffer | System.Windows.Forms.ControlStyles.OptimizedDoubleBuffer.AllPaintingInWmPaint. It will help to avoid flicker. By the way, this is a protected method, so this is another reason to use derived custom control.

You need to write you rendering method as a separate method, not inside of the handler (which will call your method). Why? Because you also need to do all the rendering on the image itself!

When you need to safe you image with overlays, you will create in instance of the Graphics using Graphics.FromImage and draw on your image using this instance and your rendering method. That's why I recommended to abstract it out — to draw on different object: the screen during editing and on the photo before you save it.

—SA