Post Interview Thank-you letter

I went for a job interview Friday afternoon - I forgot to ask for a business card so I am unable to send a thank-you email. I have read a lot of conflicting information as to whether I should send a greeting thank-you card or a typed letter. And if a greeting card is appropriate would it be okay to have s simple picture on the front - I had trouble finding anything plain.

Since I wasn't able to mail it the same day and fear that the us mail will take too long - I was going to hand deliver it to the hospital and ask the front desk to put it in the unit mailbox - would this be appropriate?

The job is at a large hospital but I was interviewed by the unit nurse manager not the HR dept. if that changes anything.

Thanks for your help, all input is appreciated. :redpinkhe