be photographed, copy, describe
寫 centers on producing a faithful representation: copying, describing, or being photographed. The common thread is capturing an image or likeness of something.
寫 is the traditional form of 写. It combines 宀 (roof) with 舄, which originally depicted a magpie and contributed the sound. The character expressed the idea of transferring or copying, possibly from the image of a bird moving its young.
Under a roof 宀, a magpie-like shape 舄 copies its own image: 寫 means to copy, describe, or be photographed.
For シャ, picture a photographer saying 'sha!' as they snap a picture, copying the scene: sha -> シャ.