I have read your support forums (http://support.sharethis.com/customer/portal/articles/446558-custom-buttons)
but it is not clear for me which code I have to define on which place. Can I directly copy the java code and write it in my child themes functions.php? I tried to do that, with my own image and my own publisher key, but then I couldn't access my website anymore, instead I got an error and had to re-upload the corrections functions.php via FTP.
Could you please elaborate on where I have to write the java code and if there are any other steps necessary to take?