The smallest 1x1 BMP file is 30 bytes, which has the advantage of still being displayable in my browsers.

Here's a 1x1 red pixel: https://raw.githubusercontent.com/mathiasbynens/small/master...

Check out the rest of the repo! This author, seemingly named Mathias Bynens, got interested in the smallest possible file in various formats.

https://github.com/mathiasbynens/small/