Making an image more abstract can increase its emotional impact, but sometimes leading with light calls for specific and concrete images. In those worship moments when the congregation should be recalling specific people or places, a photograph makes perfect sense. Photos work well when you intend to be specific and concrete.
A great example is remembering specific people in a congregational prayer. When praying for those absent, such as a particular missionary, a student away at college, a soldier in deployment, or a shut-in at home, a photo of the person might help God’s People remember them in a specific way. Similarly, when praying for leaders or politicians, showing a picture to remember by name and face might help to remember them as people in need of the Holy Spirit’s guidance.
Another time photos work well is during the offering. Since the offering is a token of how we dedicate our whole lives and an opportunity to begin thinking about concrete ways we minister to God's world, we might focus on a particular ministry through photos and videos. A slide show that highlights a youth-group trip, or photos from a South American orphanage, or video from a mission agency like Back to God Ministries International, all help people understand what this particular ministry is all about and how we can be sent out to do ministry.
Yet another time I've used photos is in preaching--photos help root Biblical passages in their historical context. These are historical stories after all, rooted in real places and real people. So when talking about Jesus calming the storm, for example, I might show a topographical map of Isael, or a landscape around the Sea of Galilee, or a photo of a first-centruy fishing boat discovered through archeology. Those were real people in a real boat in a real storm, not a fable about a generic hero. A photo helps make that reality concrete.
The key for choosing between abstractions and photos is being clear on how concrete and specific the congregation's thoughts should be a any particular moment.