from 3d coordinates to pixel coordinate of an image in a canvas group

Hi,
the problem is that I have this image on a panel and everything is treated as a GUI in a VR context.
I need to select a part of this image in order to change pixel colors, and the selection comes with a ray that pops out from the right remote controller. If I use RaycastHit I'm able to get the 3D point, but then I don't know how to get the pixel coordinates of the ones I pass over.
I don't have mesh connected so hit.texturecoord is always zero.
Any suggestions ?

Thanks,
Daniele