Here are some inspirational images I gathered while creating a concept of change on a past project. They give a strong sense of emotion and atmosphere.
When searching, I look for high resolution images on Google search, Flickr and Corbis images.
To gather abstract images type in buzz words gathered through brainstorming the concept to create a depth to your development.
After finding initial images to support your concept, images such as art and architecture should also be considered and brought in to the equation. This will give your design a sense of culture and fashion.