That's not how it works! You see, same as you I'm not really awesome at creating my own icons. I simply go for good recolors with minor adjustments or avatar icons for certain custom models/skins but when it comes to upload an icon you should keep in mind that even if it is a recolor it should still be a fine one. Yours in this circumstance seems pixelated, as for the one I posted on your bundle, you cannot download it from anywhere. You can simply make it using an advanced photo editor and the button manager. Check this:
Full Icon Tutorial (Reforged) + ¨tool¨ I hope it helps you!