10% may seem fair when the company is small but it will cause trouble later. What if the company becomes Facebook? Will future hires be angry that a short-time contributor gets 50x their equity upside? Will investors not balk at paying $500k for 10%, when someone else gets 10% common for having done a logo?

Imho, the correct way to compensate not-really-founders is with a convertible note for the market value of their services times an appropriate risk multiple, or, if you can afford it, cash. (I have actually done this.)