of course i save the file before re-opening and of course qt handles fine the alpha channel, maybe i didn't wrote it good (i apologize4my english ).
The fact is:
1- I create an image with a commercial editor and i give to that image an alpha value, and i save it: myfolder/myimage.png , and then i close everything.
2- if i reopen that image with the same commercial editor, it is able to recognize the right alpha value (not only displaying the image in the proper way but also writing it correctly in between the properties of the image)
3-instead if i reopen the image with my self made editor, i am able to display the image correctly but into the properties the alpha value is recognized as the maximum (255)

so the question was: how can i read the rigth alpha value? 'cause if a commercial editor is able to do that maybe i can also:-)

thank you in advance!