All raster images that you import basically stay raster images. They are just mapped as two-dimensional textures to a three-dimensional object for instance a N.u.r.b.s. surface, because the Rhino scene is 3D. There simply is no default two-dimensional canvas to place stuff on!
If you want transparency, you probably need a separate transparency map: a black and white, desaturated image, where dark areas result in full transparency and light areas stay opaque. A transparent PNG will probably not cut it!
Apart from the diffuse map (normal, coloured image), it additionally gets mapped to the surface or mesh to provide transparency data.
Both textures get mapped to the UV space of the surface or mesh. You may have heard the term UV mapping before. Well, you need a geometry to map onto and a UV map for placing the two-dimensional image. It’s basically an unrolled representation of the surface or mesh object to be textured. These can get very complex, but for a simple, planar geometries, the UV map is simply analogous to the geometry itself.
Here’s a way to do it in Grasshopper using the Human plug-in, available on Food4Rhino or from the Rhino
Note, how there is a mesh to be textured and a surface representing its UV map.
Here’s the image that I used.
texturing.gh (6.3 KB)