Can I specify which cards I want to receive?

No. Postcrossing is not a collector's website, and your profile should be used to write about yourself, as stated in Postcrossing's Community Guidelines.

You can indicate some of your preferences, which can help other members pick the postcard they're going to send you. However, this does not necessarily guarantee that they can or will fulfill your preferences... ultimately, it's up to the sender to decide which postcard they'll mail you.

Please keep in mind that you must still register all postcards that you receive, regardless of whether they match your preferences or not. Stating in your profile that you will not register a certain type of postcard is against Postcrossing's Community Guidelines.