Yes I should have put it here so anybody with the same problem get the answer. Excuse me.
In Preview there is a selection menu, if you keep te cursor holding the mouse buttom it opens a pop-up menu where you can select between some selection tools. The last one (which I think is called "alpha chanel" in English, although in Spanish is something like "extract using color").
With this tool selected you can put the cursor over the photo in a place you want become transparent. As you move the cursor away from the original location more area of the picture gets selected, if you go to far in your selection and want to reduce the area just move the cursor to the original position.
After you finished if you hit the return the area selected becomes transparent. I have some problems in the begining understanding how it worked, so give it a try.
I don't know which file formats suport this feature, I'm just using .png.
Obviously the tool work better if you got a solid background behind the front image. In my case it was a picture of myself against a white wall and worked really nice.
Having dicoverd this and the basic iPhoto editing I just don't need any other app to handle my pictures.
