If you need monochromatic images (ie for logos or charts) a pretty cool trick is using braille. With a little bit of hacking, the xkcd example of this repo https://github.com/asciimoo/drawille/ works pretty well :)
I'll be using this for my email signature.
Thanks for this! I was not aware of existing tools nor techniques to approach this problem.
I just tried to process the bender.original.png, and the result is not very good (img2xterm and util-say do a fantastic job with the same input). I will look into the half-block trick, and see if termbox-go supports something along those lines.
I can add this to the list, but it looks like it does not take advantage of the half-block trick to double the effective vertical resolution. (like other tools like img2xterm)