that's great! I hope it will be implemented for xterm and becomes a sort of standard. Sometimes I use "sixels" but they are hopelessly slow and low-resolution.
Not much a fan of the base64 encoding here. I'd prefer if it was the pixel binary buffer data that you output directly, pinching a hole in the terminal protocol. The fact taht it is binary does not pose problems if the escape sequence contains the size of the image.