How do I change a PictureBox's image?

Assign a new Image object to your PictureBox's Image property. To load an Image from a file, you may use the Image.FromFile method. In your particular case, assuming the current directory is one under bin, this should load the image bin/Pics/image1.jpg, for example:

pictureBox1.Image = Image.FromFile("../Pics/image1.jpg");

Additionally, if these images are static and to be used only as resources in your application, resources would be a much better fit than files.