add base url before field text content
You can render your image with a render array like this:

$a = array('#theme' =&gt; 'image',
        '#uri' =&gt; 'http://www.url.com/'.FIELD_CONTENT.'.jpg',
        '#alt' =&gt; 'Whatever',
    );


If you want that to be the content of an A tag (the link to press), render the link like this:

Link::createFromRoute($a, routename)-&gt;toString()


Is this what you need?