Correct its size with Edit Free Transform (ctrl+t) and increase the brightness and the contrast.



Change the layer's type on Soft Light and combine this layer with the previous one: Layer ? Merge Down (ctrl+e)



Apply the next selection Polygonal Lasso Tool and the command Edit ? Cut (ctrl+x) to cut out the ceiling on the background's layer.



Insert on a new layer a different sky's picture.



Place this layer lower than the walls' layer and correct its size and perspective, making use of a special command Edit ? Transform ? Distort.



Change the brightness and the contrast.



Apply next a color filter (Image ? Adjustments ? Photo Filter).



Next step includes copying the layer with the walls and placing the copy lower than the basic layer. Blur it out with Filter ? Blur ? Gaussian Blur.



Make this layer darker: Image ? Adjustments ? Brightness/Contrast.



Make another copy of the walls' layer and place it above the sky's layer.
Apply to it Filter ? Blur ? Motion Blur.



Make use of the known already color filter to the new layer, choosing Magenta color and change the layer's type on Screen.



Combine the layers with the background's parts (ctrl+e). Copy next the layer with the monks and make this copy totally white, using Image ? Adjustments ? Brightness/Contrast.



Place this layer lower than the basic layer of the monks and blur it out (Gaussian Blur).
Change the color of the halo that we've got: Image ? Adjustments ? Selective Colors.



Copy again the layer representing the monks and apply to it Filter ? Blur ? Radial Blur.