You use gimp right?
here an short tutorial:

-Open the image
-Open the layer window (STRG+L)
-select the only layer , right click , and look at the bottom of the dropdown, create alpha channel (Maybe its called a bit different in english)
-Take the Path Tool and click around the shape you want...
-if you finished you must connect the Past (STRG+Leftclick at the first path node you made)
-Now click in the main window on "Create Selection" (Or something like that)
-Press STRG+I (To invert the selection


Voila... you have an Alpha Channel...

cYa Sebastian