Yes it is safe and okay to post an image on the internet of your 8 year old son wearing just a diaper or just a diaper and shirt.
A diaper covers the kids private areas so there's nothing bad about the 8 year old being shown in just a diaper or diaper and shirt.
Here's an 8 year old boy wearing just his diaper and diaper and shirt and a boy of around 7 or so wearing just his diaper.
