My technique on how I cast shadows from the (aliased!) edges of alpha transparency rather than doing it manually by drawing and deleting faces due to it not being not feasible, requires greater accuracy or is a 3D mesh:
In Photoshop I make paths of the edges of the alpha (make path from selection), export paths to Illustrator, then in Illustrator I export to .dwg. Depending on the level of detail needed I might simplify paths in Illustrator to keep the resultant Sketchup model as simple as possible.
Its then a matter of resizing to fit and dropping this onto the imported .png and deleting the faces that are to be transparent (you might have to fix the odd edge to close a face - initially importing the .dwg file at a really large size seems to minimise this). I use the drape tool with the imported .dwg if I have a mesh that isn't 2D, then 'eye dropper' the associated and same-sized .png file and then paint bucket that onto the mesh (i use parallel projection to orientate/resize: the mesh, .dwg and .png as you usually would for the drape tool).
This technique is probably a workaround if you can't use alpha transparency at all, i.e. want to receive shadows but can't turn off transparency due to transparent materials in a model. I still use the transparent .pngs as it means I can keep a detailed edge to the transparency and have a less intricate imported .dwg file to minimise the work required as throwing shadows doesn't need extremely accurate edges. However you want to make sure that the .dwg file edges are accurate enough not to truncate non-transparent parts of the texture when you're deleting faces (I import the .png into Illustrator, lock it just for a reference and scale/move/edit the paths to ensure they're all in areas of transparency).
