The photos are not "saved" in Picasa, you are just using Picasa to view them (the same as you could use another program to view them like Windows Photo Gallery).
So you need to find out where they are saved on your disk and in which folder.
You may be able to do this while in Picasa, by looking at maybe the properties of the photo (I dont use Picasa so dont know how to do this, but I am sure you can use Picasa somehow to find out where the picture is stored.
Actually, it's quite simple to e-mail photos from Picasa. Simply open the folder you want and click on the photos you want to e-mail (holding down the "Shift" key at the same time will allow you to select multiple photos). Then, at the bottom of the page you will see a box marked "Share" ... click this box and follow the instructions. It's actual function is to upload your selected pictures to a "Web Album" which you can e-mail. Next to that box is a tab marked simply "e-mail" which will allow you to e-mail the individual photos.
A word of caution... most e-mail services limit the number of mb you can send at one time... I'd suggest starting with just two or three photos and then increase it next time to see what can be sent...