of all of your folders in Picasa. Just click 'View' then 'Sort Folder List' then you can select by Date Created, Recent Changes, or Name. Selecting 'Name' will arrange your files alphabetically in descending order and will remove the year markers.
You can sort your photos within an Album or Folder using the 'Sort By' option under 'Folder' then 'Sort by'. You can sort your photos by Name, by Date, or by Size.
Does that help?