"Objects" would be just what you expect...the very toy, book, DVD jacket, snack food, shirt or whatever other item you might be talking with your child about. If you hold up Goodnight Moon or Bartholomew Bear as you offer to read your baby's choice and she points to Ba Bear, she has just used the book as an object to express her choice. It's an intuitive thing for both parent and child to do, and can be a very convenient way of incorporating choice-making in a natural way.
Even though my daughter is a teenager and capable of reading, we still grab objects when they are the most convenient, natural option. If she's choosing a movie, why not simply hold up the DVD jackets? It's quick and involves no extra work of creating a symbol to represent each movie. The same process goes for choosing outfits to wear or music albums. Sometimes it's appropriate to be able to select from a whole library of options, but when we need a quick choice, the simplest method is to snatch up the actual objects.
Some speech therapists subscribe to a theory that we must begin with the most concrete of symbols--objects--and progress through a specific sequence towards less concrete ones. My child development background causes me to cringe at that idea; I say we figure out what works for a child where they are now and start meaningful communication at that place. Personally, I believe that by the time we realize a child is not going to use speech, they have already had a great deal of practice with objects as a means to express their choices. Most will be ready to move to some form of symbolic expression about the time their peer group is learning to talk unless they have a visual disability and can only decipher meaning by touching a 3-dimensional object. If your child pays attention to illustrations in the picture books you read together, particularly if he looks to an item you mention specifically, you know he is capable of moving to a symbol system.
Paper symbols are easy to use and store (okay, storing the symbol cards can be a bit of a challenge, but folks have come up with some clever systems over the years). Symbols come in several varieties, each with their own benefits and drawbacks. While I'll present the paper symbols from most to least concrete, please don't think your child has to "progress" through every type! One will probably have greatest meaning for your child, and that's where you jump in.
Advantages of clearly photographed objects as symbols: smallest cognitive jump from objects, easily personalized for the people/places/objects in the child's world, any item or activity that can be captured on film can become part of the child's vocabulary.
Disadvantages of photographs: complexity of photos can be distracting (ie: child studies people's faces rather than making choices), cost of colored ink, learning curve for taking/editing excellent photos, challenge of illustrating abstract ideas (ie: how do you photograph "on" without the child being distracted by the objects in the photograph used to demonstrate the concept?).
These share some of the same advantages and disadvantages of photographs. Drawings tend to be fairly obvious. Sometimes the detail can be distracting, especially if there are a large number of choices presented at once.