Render image if it exists

Say, instead of saving Boolean in database, i'll just make client side of webapp check if image exists and if so, then display it, otherwise - default image.

1. Are there any negative consequences? For SEO?
2. What if i'll not even bother about "check if exists requests"? I'll just put default image beneath the desired one and if image is not exists, then client-side js will remove the remains of the image tag.