<nav class="main-navigation stuckMenu">
    <a href="{{ .Site.BaseURL }}">
        <img class="logo" src="{{ .Site.BaseURL }}img/logo.png" alt="My Portfolio">
    </a>
    <div class="sub-navigation">
        <a href="#" class="main-navigation-link" id="about" data-scroll-goto="0">About</a>
        <a href="#" class="main-navigation-link" id="process-link" data-scroll-goto="1">Process</a>
        <a href="#" class="main-navigation-link" data-scroll-goto="2">Case Studies</a>
    </div>
</nav>
<nav class="main-navigation stuckMenu">
    <a href="{{ .Site.BaseURL }}">
        <img class="logo" src="{{ .Site.BaseURL }}img/logo.png" alt="{{ .Site.Title }}">
    </a>
    <div class="sub-navigation">
        <a href="#" class="main-navigation-link" id="about" data-scroll-goto="0">About</a>
        <a href="#" class="main-navigation-link" id="process-link" data-scroll-goto="1">Process</a>
        <a href="#" class="main-navigation-link" data-scroll-goto="2">Case Studies</a>
    </div>
</nav>

Add relevant alt tag on logo
Add relevant alt tag on logo

Ideally Alt should be site - title as it is going to site and not a static text.