The answer to this question is: What does your contract say. If your contract says nothing about copyright transfer then they own the copyright. See the thing is, the person who made the file owns the copyright of the file. Does not really matter what happened before.
What about the copyright of the things you did before. Well you own the copyright. But you see you gave the graphic designers a implied right to use them for this purpose*. So they are clear.
Next time negotiate your wishes into the contract that's what they are for. Contracts are both sides writing down how they expect things to work for future reference***.
* Even if you didn't give a implied right, by stipulating so in contract (sic), does not mean you own the copyright. Just because somebody violates your rights does not mean that you now get to own their output**. Just that now you can sue them for violation of copyright. Which may or may not mean anything in a court of law, this depends on jurisdiction. In my locale you'd almost certainly lose.
** Even if Youtube seems to work like this. What youtube does is not strictly speaking legal anywhere.
*** Note everybody always remembers the contract wrong afterwards.
This is one of the reasons that copyright usually transfers from salaried worker to company so that it does not need to be negotiated for each worker separately. This way media outlets are possible to make. This does not transfer to frelancers or contractors though.
- It took me a long time to understand that copyright is born once created. But does not project itself to new creations, it either is there at creation or not. So if you for some reason transfer the concept into a uncopyrightable medium then that thing never had copyright in first place. So copyright does not copy, you gain the copyright on grounds that you created the new one, or had the new one created in way that you control.