Hey Chris-
 your problem sounds like its been solved, but if you want a classic way of creating an alpha channel that is associated with your rgb image- here's the way in Pshop ( I don't know Elements, so I can't tell if appropriate)

if your drawing is already the background layer- use the Magic wand tool to select it
go the the Select menu, Inverse
back to the Select menu- save selection- name it something- look at your channels tab, you will now have an alpha channel for your image

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If your drawing is on a layer above the background layer, go to the Select menu
Load selection
back to the Select menu, Inverse
back to the Select menu, Save Selection, name it something...
goto your channels tab, there will be an alpha channel for your image