How do I make Slant Art from a photograph I took on a surface that raises and curves like a hill?
Example of Slant Art on a flat surface: Lego Terra Cotta Army
by Paul Cezanne
There is a Youtube video showing how they did the Lego Terracotta Army. There, they started with the "correct" image and then stretched it in Photoshop or some other imaging program. With your curved surface, that might be a bit harder; you're probably best off using a 3D rendering program like Blender (which is free but like all such software has a steep learning curve).
Alternately, use a projector positioned at your intended vantage point to project the flat image directly onto your final surface, and trace it. (I think that's the approach I'd take, if at all possible.)