Hi Tak-san,
Just from the fact that the sensor output goes to the ADC, there is advantage to the ADC sharing that ground, but I think more information is needed.
You mention "Image sensor GND---AGND---DGND". How are the different ground domains connected to each other (vias, traces, jumpers, isolated)? Are they split ground domains on the same layer? The image sensor GND is obviously for the image sensor. What other ADC related devices share ground with the image sensor? What devices related to the ADC are on the other grounds?
On our evaluation boards we normally have one common ground so managing current return paths is not so complicated.
Thanks,
Doug