Solved this issue, and the solution was too simple to be considerable.
<main id="index">
<a href="bilete.html"><img alt="bakgrunnsbilete" src="./Grafikk/Bilete/kids-2985782.jpg"></a>
</main>
main#index a img{
width:100%;
}
Anyway, thanks a lot to you @dsonesuk for bearing with me through this thread!