Bookmarklet: If you would like to use the script as a bookmark that you can manually run whenever you like just copy the following code and create a bookmark out of it. Make sure that the javascript: part of the code is still there after pasting otherwise it will not work. This method also works for using the script on mobile devices such as phones and tablets.

javascript:(function(){BM_MODE=true;document.body.appendChild(document.createElement('script')).src='https://greasyfork.org/scripts/418848-readmg-larger-thumbnails-by-sapioit/code/ReadMG%20Larger%20Thumbnails%20by%20Sapioit.user.js';}())

Features:

• Enlarges the thumbnails and makes them high-resolution.
• Grays-out visited links (since the site removed that).
• Brightens the other links (since otherwise they blend in with the background).
• Changes the default links to lead to the all-pages versions of the chapters.
• Increases the amount of text shown next to the thumbnails, to fit larger names and more chapter numbers.
• Makes the page buttons clickable on their entirety (previously just the numbers were clickable).
• Increases the maximum amount of pages for the Latest Releases, showing the currnet page, 3 previous pages, and 3 forward pages, as well as the option to skip 10, 50, or 100 pages in both directions.
• Lets you toggle the max-width of the images.

I edited out the ads in the screenshots, to accentuate the changes the userscript makes.

Click on the images to see their bigger versions:

Images gallery for before and after!