Hello. I'm new to Javascript and am looking for a simple script.

My HTML has this:
<img expr:src='data:post.thumbnailUrl' id='labelimage' style='margin: 0 8px 0 0; float: left'/>
Which displays (for example) to this:
<img id="labelimage" src="https://lh6.googleusercontent.com/-lKfLVV11BNE/UKfO_etJZaI/AAAAAAAAAHA/_vN1K5YEWHg/s72-c/TV221-1878-Tieback-Underskirt-026%255B3%255D.jpg" style="margin: 0 8px 0 0; float: left">
The part I'm interested in changing is the size of the image within the dataost.thumbnailUrl.

How would I write javascript to change the s72-c to s100?