[charles_parnot]

Yes, PNG is probably using less memory. For a few small icons, though, it's negligible either way.

[matiaspiipari]

If speed / memory is of essence, it's simple enough to pre-render this .asciimage kind of data with the code @cparnot has made available at https://github.com/cparnot/ASCIImage. I whipped up a quick commandline tool for the purpose as part of https://github.com/mz2/ASCIImage-Super-Studio:

https://twitter.com/mz2/status/579283430858465280

It could of course be rendered to a PDF or TIFF or other Cocoa supported image file format, consider that just a 5-minute hack to demonstrate the idea.