Dockerfile FROM vs Docker-compose IMAGE

I am currently in the process of learning Docker. After reading the docs and a few articles I obviously have more questions than answers. Most intriguing one for me at the moment is: what is the difference between

FROM some:docker-image

In Dockerfile and

image: digitalocean.com/php 

In docker-compose.yml

I do understand that they should grab the image and create a container from it. What I don’t understand is what happens if we will specify both at the same time, for example:

version: '3'
services:
  #PHP Service
  app:
    build:
      context: .
      dockerfile: Dockerfile
    image: digitalocean.com/php

Both docker-compose.yml and Dockerfile have images specified in them.
What happens when those images are different?
Will docker-compose.yml always win and for that one service? Will it use only this ‘top’ image?
Will they overlap somehow?
Or maybe I got it all wrong?

I did see this but I am still not sure if I understand what is going on.

Source: StackOverflow