Ok,. I just buzzed through this thread and got the jist of what your problem was. Whenever I want to do what you're doing, I use a TARGA IMAGE. In Photoshop, I would take the background eraser tool, erase a little of the portion you don't want to be visible. We're talking just a little mark, not anything more. The reason for doing this is to create an ALPHA CHANNEL. Alpha channels = transparency.

(NOTE: to get to the BACKGROUND ERASER tool, you need to locate the eraser tool on your toolbox in Photoshop. Click and hold over the button. You should get a list of three different erasers. Choose the Background Eraser Tool)

If you were to just regular erase, all you would have is a white background. With the background eraser tool, you should see a grey and white checkered background. This represents the transparency.

As opposed to taking this background eraser tool to erase the portion you don't want to be seen, I find that taking the lasso tool and CAREFULLY cutting around an object and deleting it is the best way. HOWEVER, if the portion you don't want to be visible on your image is all white, you can simply take the magic wand tool and select ALL of the whiteness at once and delete it. It will be replaced by the alpha channel (the grey and white checkered background).

Now save as a TARGA and stick it on a plane in Maya like you want. I find this to be the easiest way to solve your problem. Hope this helps.