A bad bit might have no visible effect at all (e.g. Affecting part of EXIF metadata, or a totally black pixel turning again very close to total black) or can corrupt the whole image (flipping a bit in the file header or cosine quantization table).
As a total layman when it comes to storage: Don't they use forward correcting codes to make sure a single bad bit can reliably be detected and flipped without affecting the data at all? Or are these used only for data transmission?
