From http://stackoverflow.com/questions/6018611/smallest-data-uri-image-possible-for-a-transparent-image

transparent-data-uri.gif.txt

# blank
data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7

# 10% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQkJCQC1BbgAAAABdFJOUxq9hCEcAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 20% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQoKBSBopAkAAAABdFJOUzP/NrlwAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 30% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQ0JBpcDsPUAAAABdFJOU01Ii+VLAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 40% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQwKBSTl2LsAAAABdFJOU2bkNxwLAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 50% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQsJBQqHnf0AAAABdFJOU4CtXltGAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 60% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQsIBoqV/QYAAAABdFJOU5nJNfOGAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 70% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQsIBROcrLwAAAABdFJOU75sP0btAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 80% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQsIBROcrLwAAAABdFJOU+IO4lsyAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==
# 90% black
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAADUExURQwIBRbTujkAAAAKSURBVAjXY2AAAAACAAHiIbwzAAAAAElFTkSuQmCC

# 10% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlMavYQhHAAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 20% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlMz/za5cAAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 30% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlNNSIvlSwAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 40% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlNm5DccCwAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 50% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlOArV5bRgAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 60% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlOZyTXzhgAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 70% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlOzEo46UAAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 80% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlPM0jRW/QAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=
# 90% white
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEX///+nxBvIAAAAAXRSTlPmCY+fKwAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII=