1.
绝对路径: 
this.pictureBox2.Image=Image.FromFile("D://1.jpg"); 

2.
相对路径: 
Application.StartupPath;  
可以得到程序根目录  
this.pictureBox2.Image=Image.FromFile(Application.StartupPath "//1.gif"); 

3.
获得网络图片的路径 
this.pictureBox2.Image= Image.FromStream
(System.Net.WebRequest.Create
(http://www.mxmx.com/logo.gif).GetResponse().GetResponseStream());