Before publishing promotional material of my first indie game I wanted to mark all my artwork with a legally valid timestamp.

There are two ways I know to do this:

 1 go to a sollecitor/lawyer and pay for them to certify the document
 2 use an online webservice to mark any given file/folder readable to the service

Anyone has already done this and if yes how (e.g. which website have you used? which type of solecitor have you contacted? etc..)?

Kind Regards

PS: I know that there is always the good old "send yourself a mail with a stamp and a date" but is not very strong as proof.