De Como usar o comando Assoc :
To display a list of file extensions and their associations, type assoc at a command prompt, and then press ENTER.
To display the association for a specific file extension, type assoc .<xxx> at a command prompt, and then press ENTER, where <xxx> is the file extension whose association you want to view.
To change the association for a specific file extension, type assoc .<xxx>=<file type> at a command prompt, and then press ENTER, where <xxx> is the file extension whose association you want to change, and <file type> is the program, dynamic data exchange (DDE), or OLE object you want to associate with the file extension.
To delete the association for a specific file extension, type assoc .<xxx>= at a command prompt, and then press ENTER, where <xxx> is the file extension you want to delete.
A parte em negrito é o que você gosta.
Para definir o formato .jpg como picasa, use:
assoc.jpg=C:\Path\To\Picasa
em um prompt de comando ou arquivo em lotes. Lembre-se de que ele deve ser executado com o administrador do UAC ou com uma conta de administrador no Windows XP.