Sorry if I'm asking a dumb question here, but what do you wanna do with the image after it's rendered?

Are you trying to render an image and then use that in a shader?

If that's the case, just make sure you've got "Alpha Channel (Mask)" checked in your RenderSettings, don't forget to save as a 32bit format. .TGA will work, jpg, bmp, gif won't. Don't know much about the other formats - haven't needed to.:p

This image was produced from scratch in 2mins using nothing other than Maya. (I did use photoshop to crop the screenshot, but you get the idea)I rendered a 512x512 image of 9 red balls, saved as a tga file then applied that file to the color channel of a shader.

Note that if you're using a file with an alpha channel, that the transparency channel is automatically connected too. Though it may be disconnected if desired.
(good for .psd textures that are coming up transparent when not wanted. Much quicker to kill the connection to the alpha channel than to change the image and resave it in photoshop)
Simon