Files are never 'in' Picasa 3. They're simply on your hard drive somewhere and Picasa 3 is one of many programs you can use to access them. So try looking for them without involving Picasa 3.
Assuming that they're in jpeg format (which is the default file format for every digital camera I've ever come across), run a search for *.jpg in order to find all jpeg files on your computer. (Take care to check that the search is configured to check all of your drives unless you're 100% certain that there's no point looking anywhere other than drive D). When the results appear you can click on the column headings to sort the list (e.g. by date) and hopefully locate your pictures.
You've asked us how you can retrieve your pictures from a remote server but you've not told us the nature of that server or the method you used to upload them. If, for example, you stored them in a bit of spare space allocated to your personal website by using ftp to upload them, you can simply use ftp to download them back again. However if you've uploaded them to an image-hosting website, such as Flckr, Rajce or Webshots, then the means to get your pictures back again will depend upon the way that particular website works. We need more information!