Creating an 4 channel RGBA imagen with SDL

Hi!!! I’m new in SDL. I’ve found how to make blending
to an image, but what I want is to make blending only
to a part of an image. To do so I would need to have
an alpha channel to change it manually, but I still
don’t know how to create it.

So if anyone knows how could I do it and can help me
I’ve be very pleased.

Thanks a lot,____________________________________________________________________________________
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