All historical sources manipulate events to fit their authors' political agendae. That doesn't preclude us from using them, or using them to produce a reconstruction of the past we see as accurate. For the particular topic, we've managed to reconstruct quite a bit, and if you're interested in a particularly good work on the subject, I would pick up Thomas F.X. Noble's Images, Iconoclasm, and the Carolingians, which looks at the controversy through its reception in the West.