Blank Image Encoded As Data-uri
Answer : I looked into it and the smallest possible transparent GIF image, encoded as a data-uri, was this: data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw== which is what I'm using now. 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. The smallest I've ever seen data:image/gif;base64,R0lGODlhAQABAAAAACw= update: This seems broken and doesn't work anymore as reported by @bryc, please use the other answers.