Even easier:
string myImagePath = @"images\whatever.jpg";
pictureBox1.Image = Image.FromFile(myImagePath);
Important: You have to make sure the build settings for your image files are set to:
Build: Content
Copy to Output Directory:
Copy if Newer
Note: This is
not the default setting.
If you don't have the file set to be copied to your output directory, you probably won't have acces to them at runtime.