You can fake it in the scratch you have now by checkering the color of your sprites. Take out the color of every second pixel in your sprite, so it looks like a checkerboard. You'll be able to see the jist of the sprite, and what's behind it.
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I don't remember exactly, but the Scratch Team said something along the lines of considering implementing Flash-like graphics editing (maybe even vector editing) in Scratch 2.0.
I very much hope that an Alpha channel is included for colors. Checkered pixels and ghost effects just don't do the job.
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