Your picture frame has two panes. While one pane is showing the current picture, the unseen pane is rezzing the next picture so that when the timer activates it, the picture is already loaded and shows immediately without blurring.
1. Right click on the picture frame, choose edit, and then select the content tab in the edit window.
2. Drag pictures or textures from your inventory into the content tab.
3. Delete any pictures or textures you do not want from the content tab.
4. Click the general tab, and on the description line type in the number of seconds you want the picture to show before changing to the next.
Note: If you set the number seconds to too short a duration, there may not be sufficient time for the next texture to load before the timer activates it, and it could appear to be blurry for a few seconds.