I know this question is better suited for an attorney. The only reason I haven't reached out to one yet is simple: even if I find out I retain ownership and full rights to the work I have done, it won't change anything. I'm not going to request they stop using my work.
I'm actually thinking more along the lines of my portfolio. Can I, without asking them for permission, put the work I did in my portfolio? Due to that element alone I may just go ahead and put an email out to an attorney or two.
Oh unless you're under NDA or some other contract that prevents you from showing the work in your professional portfolio, then it's usually understood that you'll be doing this even where your employer retains the rights.
You are probably good to go ahead but if you're not sure then why not just discuss it with someone from your former employer as a courtesy, since you're leaving amicably?
I don't think you'd need to involve a lawyer over this unless they decided to fight you.
You were paid by this employer to do this work. You designed a website and graphics that were used by their business to promote their business. That is your contract, you were their employee. Any work whether it be design, arts, or working on cars- while employed by that business is owned by them- if it was done for that job. If you design a graphic on your own computer at home, never show it to them- and then after leaving you decide to use the same design somewhere else that is fine. It is not okay to use one of the designs you made for the former employer again. But honestly this is not something you should ask an internet forum about, you should seek legal advice from a legal professional.
For example, take Magic the Gathering. Early MtG cards, copyright was owned by the artist. They had to be pad every time a card was printed. About the late 90's, Wizards changed their policy. Wizards now owns all artwork for their cards, rather than the artists.
"My employment for them has never included a contract. "