const gallery = document.getElementById("gallery");

for (let i = 5; i <= 47; i++) {

    let img = document.createElement("img");

    img.src = "photo" + i + ".jpg";

    img.alt = "Centre of Excellence";

    gallery.appendChild(img);

}

<div class="gallery" id="gallery"></div>

<script>

const gallery=document.getElementById("gallery");

for(let i=5;i<=47;i++){

    let img=document.createElement("img");

    img.src=`photos/photos${i}.jpg`;

    img.alt=`Centre of Excellence ${i}`;

    img.onerror=function(){
        this.src="photos/photos5.jpg";
    };

    gallery.appendChild(img);

}

</script>

</body>
</html>