>But don't you think the shaping of his hair is different ?>from regular Ssj?

Hi
Yes, in that cel it looks different. It will not always look
very different though. 

I beleive this cel might also be ssj2-
http://anime-online.com/GokuTheGenius/dbzcelgallery1_files/GokouSnarl_S_.gif

They can go to different levels of ssj2. If you watch the
Gohan vs Cell fight you can see his hair go from Yellow
to gold to almost a pale whitish-yellow, it all has to do
with how powered up they are, you can also see the shape 
change to like inbetween ssj and ssj2 (semi-ssj2 o__0). 

With Goku
the hair can also be ssj shaped but have a very gold color
making it ssj2. Basically with Gokou I think if you have one
that looks like it is from the Majin Vegeta fight or has a 
couple extra small bangs, then just go watch and see if it
is before or after he turns ssj2.

Reguards
    Jamal <~~Has looked at far too many DBZ cels over the 
past 2 years.