Picaboo is capable of sorting your images by the file name of the photo, or by the date the photo was taken. This is possible for both the bottom row of imported images (Media Tray), as well as when you're importing new images into a project.
In Edit mode, click on the Advanced button near the top right side of the program window. From that menu, select Preferences. The Preferences options will appear in a pop up window.
At the bottom of that window will be the option to sort the images either by Date Taken or Filename. Once you've selected the option you would like, click on OK and the images in the photo tray will sort themselves accordingly.