Tx, yes I guess I could let them as they are.
I was now trying to "Make all the semi-transparent black layers into fully-transparent layers.", in Photoshop (I don't use it normally, I use PhotoImpact). Found some problems finding where I can change the transparence value for the Alpha channel (or I should look for layer transparence?

). I change the alpha channel opacity but nothing...I keep trying
Thanks for all your advices and kindness.
EDIT: Ok, I have found that is not the opacity control which I should change; it is the grey color of the alpha channel, so if all is black, all is 100% transparent!