Are you wanting to make a form that has an odd shape? As in, you want to have the transparent parts of the PNG not be clickable as a form? If so then you are going about it the wrong way. If you were to put an image on the form and set the transparent properties on the image, then all you will do is see the form behind the image.
If you want to have a form that is the shape of the visible area of the PNG then you need to create an oddly shaped form. This is done by using regions and SetWindowRgn.
Luckily there is a handy tool out there for this: Gabes OddForm Assistant. I have used this to great effect a number of times.
In order to use it you will need to save your PNG as a bitmap and then load it into the utility. Once it is done it will give you the code required to make the form. You can just have a look in the pas file it creates and see how it is done, tinkering if you feel the need. You can either use the form directly or graft the code to where you need it.
Hope this helps.