An overall photograph places the affected frame, reveal or sill in context. A close view adds surface detail. Say whether one window or several are affected and describe wetness you have actually noticed. Mention earlier leaks, repairs or cleaning without assuming that the visible position explains the cause.
If a curtain, blind or furniture restricts the view, explain it rather than disturbing the affected surface. Describe the material where known: painted plaster, a frame finish, sealant or another surface. Those details help establish suitable assessment and treatment rather than a quotation based solely on a dark corner.