There are two ways in which to view pictures on the Photos application;

Picasa or Flickr Account
1.     From the Home bar select All Apps > Photos.
2.     Select Add an account.
3.     You will have options to create/add and account for Picasa and Flickr.
4.     Once an account is selected you have access to all pictures that were uploaded to that account.

Connect a Local Media Device (USB Device)
1.     From the Home bar select All Apps > Photos.
2.     Select Switch to Local media.
3.     If a USB device with pictures is connected you will be able to view those pictures.

If you have difficulties viewing pictures from Picasa please see;
Unable to view pictures from Picasa on my Revue

If you have difficulties viewing pictures from Flickr please check http://www.flickr.com/help/