replace(/(.*)\/(.*)/, "newPath/$2");

<img id="mainImage" src="default.jpg" alt="Main image">

<a href="#" onclick="document.getElementById('mainImage').src='large_image.jpg'; return false;"> <img src="thumbnail.jpg" </a>

<img src="path/to/image.jpg" onclick="window.location.href='>
<img src="path/to/image.jpg" onclick="this.src=location.href='>
<img src="path/to/image20-10.jpg" onclick="this.src = this.src.replace(/\-(\d+)/g, (match, p1) => { return parseInt(p1) + 1; });">

let newStr = str.replace(/\-(\d+)/g, (match, p1) => { 
return parseInt(p1) + 1; });