I don't want to spend time on this as there
was already many threads on that subject in the 
past so you can look in the archive.

But yes, the scan that you make of a cel is 
copyrighted by yourself. Whatever you own the
copyright to the character or not.
That mean that someone can't simply takes your 
scan and use them without your agreement.

Naturaly, that doesn't mean that you can do 
whatever you want with your cels : It's not 
because you own a cel that you can use it to 
produce and sell a poster for example. The 
copyright of the character/work belong to the 
studio and when they sell a cel, they don't sell 
any kind of licenses.

Cheers,
Yann Stettler