how do I INSERT A PICTURE ON WINDOWS FORM APP WHILE IT IS EXECUTING?

In addition to the answer by BacchusBeale:

Using the control System.Windows.Forms.PictureBox is just one of the way to add an image. I only would like to emphasize: each and every image or control is always added "while it is executing". Even if you do it using the designer, it is actually added "while it is executing".

Here is my general advice: if you know how to add anything using the designer but not sure how to write appropriate code by yourself, first do it under the debugger and then look at the auto-generated code. In case of Forms, it is added to your project, in case of WPF, it is not, so you would have to find it under your project's directory.

In you case, for example

Control parent = // it could be any container control: Panel, TabPage, Form
PictureBox picture = new PictureBox();
picture.Image = // whatever image you want to show
// set position, dock and other required properties...
// add it:
parent.Controls.Add(picture);
// alternative way of doing the same:
//picture.Parent = parent;

Generally, this is the way to add any control.

—SA