Well, first you describe the picture... answer the usual questions of who/where/when/etc. Then explain why it is your favourite. Does it make you feel a certain way? Does it remind you of an event or those people? What is the background story? Could it also have a special meaning to someone else?

example: if you have a beautiful photograph with your mom you can write about it.when/where did you take that,who else was with you ....why do you like that picture..how do you feel while looking at that picture etc

i can help you.....if you intend...Greetings