Dev tools are built into the browser.
Right click and Inspect and element on the page.
Or in Firefox: Settings > More Tools > Web Developers tools
Chrome:
Jekyll forums let’s you expand images.
Mouse over.
Click the expand button in the bottom right.
Click on the image to make it big.
The code is like this - either in CSS file or style property on figure element.
figure {
float: right;
width: 60%;
}