Sounds like your power management settings are powering your monitor off after a period of time, thus stopping your screen saver from showing your pictures. Simply go into your power management settings and set the monitor timeout to never and you will be good to go. HOOAH!!!
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.
My pictures that I want as screen savers only stay on briefly. how do i get them to stay on till i shut computer down?
Are you referring to your screen saver or desktop background? When you set a screen saver and move the mouse or touch a key on the keyboard the screen saver will cease. You can also have power settings set to turn your display off after a period of time.