Base64 Images in Email Signatures

Base64 encoding turns an image file into a long string of text that can be placed directly in HTML as a data URI, so the image travels inside the email rather than being fetched from a server when the email is opened. It is tempting for signatures because it avoids hosting and avoids the download images prompt in some clients. In practice it is one of the least reliable ways to include a signature image. Gmail does not display data URI images on the web or in its mobile apps, so the logo simply disappears for a large share of recipients, and Outlook for Windows does not display them in versions up to 2016 and supports them only partially in later versions. Base64 also inflates the image by roughly a third, and that weight is added to every email and to every reply that quotes it. The reliable options are a hosted image, linked by an absolute https URL, which is what most signature tools use, or a CID embedded image, which attaches the file and references it from the HTML. CID images work without a network request but add attachment weight and can show up as attachments. For signatures sent to a broad audience, hosted images are the standard.