Could be 2 things here. A. you are using the wrong thing to do what you want, the screensaver will only stay on till you move the mouse or type something, to get a picture on the PC all the time, you need to set it as a background. There are rotating backgrounds you can set in Windows 7 that change, but you need to check the settings on how to configure that.
B. If your monitor is shutting of like GOLGeek suggested, do the power management.
If you want to play a slideshow of your own pictures, Picasa can create it and will rotate as many pictures as you have if you want all day long.