Image plane Thickness

[micheus]

If all the problem is related to the image plane thickness, that is too easy to resolve: select the top face that contains the image mapped and use Extract command and accepting it without any mouse movement.

That will create a single face already UV mapped and assigned to the initial image. So, you can throw out the initial image plane.

Image plane has a thickness because - you know - Wings3D works with manifold meshes. Extract was something probably created to be used in situations like that.