Generating oauth signatures is kind of complicated, and where a lot of
people on this board have trouble.
You're best off finding a good oauth library to generate the oauth HTTP
call for you. It will generate the appropriate oauth headers and signature
for your HTTP request, making your life much easier.
It looks like you're using Ruby?
John Bunting (works for Tumblr, and often replies on this board) has some
kind of Tumblr Ruby wrapper here:
Not a Ruby programmer personally, But I'd look into those.
Maybe Mr Bunting has something to add?