Picasa provides two sorting commands that you can use as needed to make it simpler to find pictures. First, you can set a sort order for folders in the Folder List. Second, you can specify a sort order for the images within any folder.
Sort folders in the Folder List. To sort the folders within each collection, choose Creation Date, Recent Changes, or Name from the View > Sort Folder List By submenu.
These are examples of the three folder sort options. Note that each collection is sorted separately.
Sort within a folder. Choose a command from the Folder > Sort By submenu (Name, Date, or Size) to set an order for the images in the current folder. (You can also choose a Sort Folder By command from the Actions button menu.) Every folder can have a different sort order.
Folder sorted by Name (filename)