First, you must make sure that you upload the picture directly to your pic of the day gallery.
If you move it there from another gallery then it will show up at the time you originally uploaded it to the other gallery (click through the PAD metagallery and it will probably be on page 2, or 5 or wherever.)
If you have uploaded it directly to your pic of the day gallery, it still might not show up for a while (although it says the page is refreshed every ten minutes it can even sometimes be a couple of hours) so check the next page and the page after.
Finally if that isn't the case (ie it definitely isn't showing up) then you have been unlucky with a bit of a glitch (which has happened) so then email
help@pbase.com and let them know.