use images from the public folder instead of looking up within frontend

[maxime1992]

Thanks to PR #51 (with docker), I noticed a weird thing in code:

I decided to use the local images directly from the frontend folder instead of using the ones from backend/public.

This is because while we develop, we don't have to build the frontend and put it in backend/public.

But with Docker, we only keep the backend folder and thus, we loose all local pictures.