Thing about ASCII pictures in the chat is, not everyone will have the font set which aligns the text just right. Doesn't matter if you solve your double spacing it's still going to be missaligned for those with a font which characters aren't evenly spaced.
mIRC's default fixedsys is good for the ASCII picturing because every character in the font are spaced exactly the same. Many users, including myself, use a different font which character spacing is different depending on each character. For instance, even if, to you, the thumbs up would look straight to people like me it would still be out of wack because the fonts character spacing isn't exactly the same for each character.
If you ask me the ASCII picturing in the chat is just a waste of time anyway ;(.
CtrlAltDel - The simplest way to preserve the spaces, is to put your ascii thumb in a text file and use the alias to play it:
alias thumb play thumb.txt
You're right, that's about the only easiest method to use that preserves the spacing. That's always been odd to me that playing does what saying doesn't.