I have a nice photo of a painting, but poor lighting condition. The light is falling from right, so there is a gradient in light. My question is, can I level it out to remove the gradient as if there is a perfect all around light falling on the image? I add the photo.
Basically yes, but it may not be easy to get it right.
Create a curves adjustment layer and add a mask to it. Fill the mask with a horizontal gradient. Then adjust the curves lighter to get rid of the shading. If you're not familiar with curves, this can be quite tricky.