The Portal Style Guide contains a lot of helpful information for creating a rotating Flash banner (like the one used on the NASA.gov homepage).

Specifications cover:

• Graphics Standards
• Bitmap Standards
• Text Standards
• Titles
• Call to Action
• Stars, Feature Image and Text Transitions
• Actionscript Timer Function and Anti-slip Layer

+ Download the editable Flash file (flash_standards.zip)

Read the Creating Flash Banners section in the Style Guide for more details.
+ Online version (NASA Only)
+ Page 33 in PDF (click to download)