I have used cardboard from boxes, they are firm enough. Then buy your felt, put it over the cardboard and bring it around to about 4-5 inches on back. Take duct tape, pull the felt tight and then tape. As far as what to put on the back of the pictures, use, I think, Viva paper towels. They will stick to the flannel. Sometimes when you use felt on the back of the pictures, it is too heavy to stick. Betty Lukens has "learning" felt pieces already made. There is another company too, but I can't think of their name.
I am using felt board with my daughter a lot (she is 3.5). We use it for Bible stories (I have a ready made flannel Bibile set), for math and just for free play. She loves it. I made pieces both ways- out of flannel pieces as well as out of cardboard with small pieces of flannel stuck to the back of it. I have a set of two flannel babies with different accessories and she sometimes plays with them for hours. I am a fan of flannel boards
MIL had another idea for you. She said to print your pictures on iron on transfers and put them directly onto the flannel instead of having to attach the flannel to paper. She said she's seen many people do that with great success.
