WordPress.com comes with a pre-selected range of 200+ themes, some of which work really well with portfolios of different types. If you do a search for the word "portfolio" in the theme directory, you can check out a few potential themes:
Third-party or custom themes can't be uploaded on WordPress.com. You can learn more about the differences between WordPress.com and WordPress.org here.
If you want to keep your domain name and switch to a self-hosted site, I suggest you either transfer the domain to another registrar or
leave it registered here and update the name servers to point it to your new host.
If you need help with anything, just let us know.