Which parts? I image edit and do my png->gif conversions in photoshop, and the desktop version of COC2 will accept either format, even though it requests pngs. Gifs are super small, so you can go way over the 100x100 suggested limit, and the result generally looks better in game.
The actual editing process necessary to selectively target border pixels to darken them is frankly something that would require a tutorial. I really just wanted to point out that saving as gif will result in smaller file sizes, so you can include way more in it. Less fuzzy.
I don't know why they specified 100x100 - possibly a design goal for uniformity's sake, or just an estimate for how big you could probably get a png without it going over the file size limit. Here's three game-ready versions of your image, two of which are up to 250x250:
View attachment 19286View attachment 19287View attachment 19288