Unless a contract states otherwise, the default position, in law, is that the copyright always remains with the creator of the artistic work, so physical art should come back to the originator really. So beyond that, it depends what your agreement was.
As Levi says, if they commissioned the work for a specific purpose, then even without a contract and with copyright retained by you, ethically it would be wrong for you to reuse them for anything else – with the exception of the copyright fair use clause that allows for such uses as being shown in a portfolio, or as an exemplar in an academic work.
You can, obviously, write a contract that hands over the physical artwork and/or the copyright – with the requisite remuneration.
Whenever illustrators send me original art, once I have had it scanned, it goes back to them as a matter of course, unless they don’t want it back or I have asked nicely. Once I commissioned a well-known illustrator to do a book on cycling for me. I did the original sketches to show him what I needed. I loved the end result so much, that I asked if I could buy it off him just to hang on my wall. In the end, he let me keep it, as I think he felt the fee was enough to warrant it, but it has to be that way. It is his by default.