Lately, I've had several cases in which a picture communications system has been discussed in order to help children communicate who aren't using any other form of communication yet (signs, words). I recently listened to a webinar about young children on the spectrum who really benefit from pictures systems. Children with severe expressive language delays and/or apraxia of speech may also benefit from a picture system.
Depending on the level and circumstances of the child, your pictures may vary from child to child. One of my kiddos has visual impairments and needs great big 8 x 10 pictures that are simple and not cluttered. One of my kiddos could benefit from 4 x 6 pictures. Some children needs real digital camera pictures, others need cartoon images.
For my kiddo who is low level (low receptive and expressive language) - I will use the pictures to "teach" to start out with. As he gets his bottle of milk, a picture of milk should be placed next to it as it's labeled "milk." When he grabs his ball, a picture of a ball is shown with the label “ball." Although it seems very basic, this child has not yet associated words with objects, a skill that has to be met before he will ever say "milk.' If this is done consistently every day, he may begin to use the pictures more communicatively such as bringing a picture of his milk to mom in order to request milk. This would be the ultimate result!
For a higher level child, you may use pictures to offer choices or create a visual schedule. When you offer choices ("do you want milk or juice?") it is best to either have the actual object in your hands or pictures of the choices you are offering. This gives a visual cue in addition to your auditory clue. All children like to know what is going to happen next and a picture schedule does just that. If the child can see that he gets to play outside after lunch, he may be more at ease during lunch. Remind the child - "First we eat lunch and then we play outside."
Use Boardmaker, use Google images, use cameras. Laminate the pictures so they last longer. You can make the pictures magnets and place them on your refrigerator. You can Velcro the back and make a strip that the pictures go on...the child can rip off the picture after the task is completed.
Every child is different as in any case, but if what you are currently doing isn't working...maybe pictures should be added to your bag of tricks next! Good luck!