This is an old school method widely used in the 80s (e.g. on ZX Spectrum): each 8x8 block had a foreground (ink) and background (paper) colour, monochrome but using the colours well would deliver excellent pictures for title screens.
Now, One-Off GFX is based on that method but could also have more than two colours in a single block (at the expense of extra chars).
So -differently from other image compression methods- it doesn't really matter:
It just depends on how many colours are present in each 8x8 block; better keep it less than 4 and possibly just 2 in most of the blocks choosing wisely while drawing or editing the pic. A good drawing tool to improve monochrome skills is Multipaint (choose c64 hires mode)
Ok we have an optimised gfx, save it as .PNG, what now?
Download this cart and use it as converter tool:
How to use:
Use it in your project/game cart:
A -not so well optimised- example of a comic:
The comic example cart is here
Wide,Tall, Striped and Pinball modes
See comment below
(More to mention... using smaller blocks as sprites, transparency, etc...)