According to
this post Members should already have unlimited length for signature, just no colors yet.
Accoring to what's written in account settings there is still 4000 characters limit.
I would use
cryptogames logo code as an example to learn how to do it. Yours picture is two colors too and much easier thous it should be easy to modify this code:
[center][table][tr][td][center][tt][size=2px][color=#3f80bd]
▄▄▄████████▄▄▄
▄██████████████████▄
▄██████████████████████▄
█████[color=orange]█████[/color]██████[color=orange]█████[/color]█████
████[color=orange]█████████[/color]██[color=orange]█████████[/color]████
█████[color=orange]█████████[/color]██[color=orange]█████████[/color]█████
███████[color=orange]████████████████[/color]███████
████████████[color=orange]██████[/color]████████████
███████[color=orange]████████████████[/color]███████
█████[color=orange]█████████[/color]██[color=orange]█████████[/color]█████
████[color=orange]█████████[/color]██[color=orange]█████████[/color]████
█████[color=orange]█████[/color]██[color=orange]██[/color]██[color=orange]█████[/color]█████
▀█████████[color=orange]████[/color]█████████▀
▀██████[color=orange]██████[/color]██████▀
▀▀▀████████▀▀▀[/tt][/center][/td]
[td] [/td]
The best ASCII characters which I recommend to use for your BBCode images are: " ▀ ▄ █ ▌ ▐ ░ ▒ ▬ ▂ ▃ ▅ ▆ "
But mostly I use " █ " and always in a size 2pt. Which is most compatible for different web browsers and different operating systems.