Using your original method, when you've painted out with white.
I would just click the white with the magic eraser to get it transparent.

Then you can export this as a PSD instead of a GIF. (Nothing supports transparency better than a photoshop file!)

When you bring that file back into photoshop it will have kept the transparency.

What I would do...
If you're more comfortable with painting than tracing round. Go into quick mask mode.
Paint your mak onto your building. Use a feathered brush round the edge if you want soft edges.
Out of quick mask and cut'n'paste into the new document. It saves exporting and importing.

regards
pk
www.planit3d.com