Hi Rose
Can't help you with Mickey but for actors and actresses you will need to write a letter to their agents, just a short note to say why you admire them (long letters are usually binned) and that if at all possible you would like a dedicated signed photo. Not many stars will send just a photo without dedicating it to someone because these can be sold on for a profit otherwise (I get them but I sell them as part of my business but I also have to pay for them although I get to meet some of them personally which can be nice sometimes). Enclose a stamped self-addressed A4 size envelope if writing within the UK, if outside the UK you will need to enclose an International Reply Coupon for the country concerned (usually the USA). Also be prepared for a long wait as the people concerned can be very busy.
Good luck!