I am not a graphic artist, but we do this all the time.
Just make exactly what you describe, a transparent image and upload it to the groov Image Library.
nothing_shapes as svg.zip (850 Bytes)
The Forums will not allow .svg files to be uploaded, so I have zipped up two of them that I use in my groov Project.
Hopefully looking at these in your graphics program will show you how they are made.
In use, I simply place these over the top of the shape I am looking for action. I use the groov Z-order, and can even attach controller tags to them.
If you don't want transparent, then make the shape the same color as the groov screen background, then use the image gadget to switch between the image that's the same color and the image that is what you are wanting to see based on the tag value.
Let me know if you have any more questions on this and I can take some screen shots from groov to show how we set this up.