Themes - please help

I wonder if you can help me I have been using Lasyk myThemeCreator
professional 1.318 to create a theme for my Sony Ericsson mobile phone.

I have noticed in some themes that all the space given to create an image is
not used, and the unused area is transparent so the image underneath shows
through.
In one theme someone used a circle as the desktop highlight and the area
around the circle was transparent so the image underneath shows through.
Whenever I tried anything like this I always ended up with a white area
around my circle rather than being able to see the image underneath.

I think I made the background of the image transparent when I created it in
PhotoShop (assuming I had done it correctly) but when I load it into the
Theme Creator it displays a white area rather than being transparent.

How can I make part of my image transparent so the image below will show
through?
Do I need to do something in the theme creator software?

I hope you understand what I mean as I have not explained myself too well.

I hope you are able to help me.

I appreciate any help or information given.

Thanks!

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