base64-image.de encodes on its servers: its own page says images are uploaded over HTTPS and then encoded to a Base64 string there. A browser-based image to Base64 tool reads the file and produces the data URI on your own device, so the image is never uploaded.
base64-image.de is a handy image to Base64 tool, and its own description says the images you drop in are uploaded securely over HTTPS to its server, where each one is read and encoded to a Base64 string. Encoding an image to Base64 is pure data conversion that a browser can do on its own, so a client-side tool reads the file locally and outputs the data URI without any upload at all.
Local encoding is the better fit when the image is a private asset you have not published, an internal icon, or a screenshot, because it never has to travel to a server just to become a string of text. You get a ready-to-paste data URI for an img tag or a CSS url() value, generated entirely on your device, with no account and no wait.
No. The file is read and encoded locally in your browser, so it never leaves your device.
A Base64 data URI you can paste directly into HTML or CSS.
No account limit; large files are bounded only by your browser memory. Data URIs suit small images best.