hey :)
i doubt that is genuine, as there is a lot of fakes circulating that look like that.
even if it is the original photo, there are many writing autographs for him, or using one of the pens that just signs hundreds of posters or whatever in a go. if you look at more recent autographs, you'll see that Dan's is a lot more scruffy and wavy now.
however i might be wrong. i wouldn't worry about it, any response is a response :)

Wrote a letter to Daniel Radcliffe in November 2005 and received autograph picture 3 days before Christmas

Daniel Radcliffe
ARG
4A Exmoor Street
London W10 6BD
UK

http://surfmypictures.com/image/784780fcc17dfa59/v49ov.html