CSS to override Jetpack's default sharing icons.

override-jetpack-sharing-icon.css

body li.share-facebook a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-tumblr a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-twitter a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-google-plus-1 a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-linkedin a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-press-this a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-digg a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-stumbleupon a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-reddit a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-pinterest a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-pocket a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-kindle a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-email a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body li.share-print a.sd-button > span {
	background-image: url(http://placekitten.com/16/16);
}

body a.sd-button.share-more span {
	background-image: url(http://placekitten.com/16/16);
}

@media only screen and (-moz-min-device-pixel-ratio: 1.5),
	only screen and (-o-min-device-pixel-ratio: 3/2),
	only screen and (-webkit-min-device-pixel-ratio: 1.5),
	only screen and (min-device-pixel-ratio: 1.5),
	only screen and (min-resolution: 120dpi) {
		body li.share-facebook a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-tumblr a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-twitter a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-google-plus-1 a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-linkedin a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-press-this a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-digg a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-stumbleupon a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-reddit a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-pinterest a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-pocket a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-kindle a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-email a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body li.share-print a.sd-button > span {
			background-image: url(http://placekitten.com/32/32);
		}

		body a.sd-button.share-more span {
			background-image: url(http://placekitten.com/32/32);
		}
}