ArcGIS Explorer 440 includes a new set of cartographic point symbols, with many to choose from. These are billboarded, so will always remain upright and oriented properly, and will grow slightly when you hover over them. Create a Note, then right click the Note to edit its properties. Choose Symbols to see the entire palette:

You can add your own point symbols from local files by clicking Add a Symbol on the Select Symbol dialog. Browse for a local file, or enter a URL pointing to a symbol you want to use that is available on the Internet.

The symbol will automatically be resized and reformatted, and stored in your local symbols file. You can view the contents of your local symbols file by clicking Styles on the Select Symbol dialog.

Your local symbols style file is located at:
C:\Documents and Settings\<user_name>\Application Data\ESRI\ArcGIS Explorer\Styles

Hint: If you want to add custom point symbols to your map, and make them available to anyone you share the map with, put the symbols on an Internet accessible location, and use a URL to add them. When the next user opens your map, the custom symbols will be retrieved via the URL, and automatically added to the user's local symbols.

For more information, refer to the Help topic on adding a new symbol.