Hi cmat,
If you use the Email option within the photo gallery it would open the default email program set on the computer.
If you are using the yahoo web mail then you will need to add the picture to the email option that is available within the yahoo web mail.
To use pictures as screensaver, try the below steps.
1. Open Screen Saver Settings by clicking the Start button, clicking Control Panel, clicking Appearance and Personalization, clicking Personalization, and then clicking Screen Saver.
2. Click the Screen saver list, and then click Photos.
3. If you want to specify a location for the pictures you want to use in your screen saver, modify the slide show speed, or configure any other screen saver settings, click Settings. After you make your changes, click Save.
4. Click OK.
Use pictures as your screen saver
http://windows.microsoft.com/en-US/windows-vista/Use-pictures-as-your-screen-saver
Hope this helps!
Halima S - Microsoft Support.
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