what is the condition for image theory

in antenna analysis.some dipole antenna is there above ground level. then thay are assuing one image is present bleow ground.
any condition is ther for this assumption. are always we have to assume?

I think you refer to a λ/4 monopole (ground plane) antenna, which has the image form a λ/2 dipole that radiates only upward.
For a λ/4 ground plane antenna always we have to assume there is an image below ground.

I think the underlying assumptions are generally related with the ground. To make the image theory valid, the ground should be electrically large and own good conductivity. Otherwise, modifications are needed to take diffraction and loss of the unperfect ground into account.