Good news. You do not have to limit yourself to postcards. Full letters and photos are accepted at all Arizona State Prison Complex facilities including during the reception phase.
Reception can feel like a communication blackout from the outside because visits and phone access are often restricted while your boyfriend is being processed and classified. But mail continues to move during that period, and a full letter is absolutely something he can receive.
This is actually one of the most important times to write. Reception is disorienting and isolating. Your boyfriend is going through intake, medical screening, classification interviews, and the general uncertainty of not yet knowing where he will be permanently housed. Getting a letter during that stretch is a genuine morale boost and reminds him that someone on the outside is present and thinking about him.
Write the full letter. Include photos if you have them. InmateAid can handle the mailing with your address kept off the envelope entirely, which is worth considering since reception facilities process a high volume of mail through multiple sets of hands before it reaches the inmate.
Once he clears reception and gets his permanent assignment, communication typically opens up further with phone access restored and visits becoming possible. Until then, letters and photos are your best tool and there is no reason to hold back.