How do I reverse the order of photos in a folder or in picasa 3?

I have taken loads of photos and want to make a stop motion clip with then. However for the fist half I have taken the pictures backwards and want to reverse then so it's right in the video clip, So how can I reverse the order for a number of photos in just a folder on my computer or even in picasa 3 (that is the programme I am sing to make the video)?