you can take hidden Mickey pictures any time, the pictures will remain in your inventory until you delete them, some folks put theeir Mickeys on a wall to help others with the quest, I put my Mickeys in a in a photo album so I could leave a caption telling which room they are in
one thing though, if you are going to take a picture of a Hidden Mickey, may as well start the quest, it's expensive but I think you earn more than you spend when you are finally done but I don't remember, it will cost about 2000 credits to get all the Mickeys
I took a picture of Doanld walking by the window over the photo supply shop in Central Plaza
still welcome to try a ride if you have the inclination