In PowerPoint 2003, this option was located under Format Picture however the latest version has moved things around. To access the alternate text . . .

1. Right-click on the image.

2. Select Size and Position.

3. Select the Alt Text tab.

4. Make your changes to the Alternative text.

5. Click the Close button.

NOTE: In Word 2007 this option is under Size and in Excel 2007 you will find it under Size and Properties.