Problem with clip to polygon of a raster (png)

Hi there,
I have a problem I can't sort out. I've applied the same methodology succesfully in the past but can't seems to find a solution this time.

I had 5 pictures that are not touching each other, however I have merge them to a single png. My problem is that the background is black and I would prefere transparent. I can't select the symbology as black being transparent because the picture is a black and white so it would removed all the black.

Here is what I get:


So decided to use a polygon shapefile to clip to the polygon (orange) (ticking the box "use input feature for clipping geometry")

Then i get to either:
-if I don't change the output file and it go straight to the database, it take an hour but seems to sometime work but then If i rexport a shapefile the black background came back
-if i choose a layer I want "image_clip.png" also try jp2 and tiff... the computer do things for hours and never finish the output also it has create "image_clip.png" but this one is clip but all black inside the boundary.

Any idea what am I doing wrong? Why do I get an output when it go to the database but nothing when I select my own output?

Here is what I would like to get but I want some shapefile and not something integrated within the database.


Hope this is not too confusing.
Thanks

Using Arcgis10 on win7 32bit.