I’m very new on HTML & CSS. This is a site developed by my friend who is unable to solve the issues now. There are 2 issue which are linked to each other.
When we zoom In/Out the logo image changes its size completely, and if we change to mobile view the logo is stretched.
To make an image maintain its aspect ratio the height should be set to auto.
But you have a strange width setting too, 200%, there are few instances when you should want an element to be more than 100%.
<aside class="off-topic">Your Banner1.jpg image is a bit on the large side, it could do with optimising to speed up load times.</aside>
Yes, something like width: 100%; height: auto would be better, then control its size via the width of its parent: .logo possibly using a width and a max-width.