If you need a transparent image 1x1 pixel, just set this data uri as src
default attribute (keep the ///
parts, it encodes byte 255, not a comment).
data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==
This is instead a base64 encoding for an image 1x1 white.
data:image/gif;base64,R0lGODlhAQABAIAAAP7//wAAACH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==
Otherwise you could set data:null
and save ~60 extra bytes for each image.