how to upload existing captured image of CentOS on FOG?
I’m not sure if that’s a typo or poor English… no offense.
You can not upload an already captured image of anything to fog, unless the capture was done with another fog server.
If you’re simply asking how to capture an image of a CentOS machine, this is pretty basic. Network boot the machine, register it with FOG. Create a image definition in the web interface. Assign that image to the just-registered host via the web interface. in the host’s basic tasks area, choose upload. Then network boot the host again.