Or, if you want an easier way, left-click the start page to focus it, hit ctrl + shift + i to open the developer tools. Make sure you're on the "Elements" tab. Then, at the top left of the developer tools, click the button that's a dotted square with an arrow.

Then, click the speed dial you want to select. Then, in the "Styles" section below, you should see:

background-image: url("chrome://bookmark-image?id=11&timestamp=0");

for example with that chrome URL shown as a link. Click that link and it'll open the image in a new tab.