Answering to myself, there is a “filltrans” script at silent9.com for gimp.
second answer , i found a crude method thinking that Animall doesn´t make too much sense to animate a character, The solution:
after cutting my character image as plane to pieces,(legs, arms etc) subdividing the imageplanes till having enough vertices to "erase° smoothly the contour eliminate the all alpha layer, decimate after: once to remove at list half of the (too many contour vertices,> planar decimate > smooth vertex to get rid of the now very jaggy edges of the planes, go to uv editor to examine some texture discrepency, Now the transparent pixels are out! (just a little aura when selected as opposed to fat transparent pixels when unselected)
Also! when you use the unsubdivide modif, on a plane mesh , each iteration rotates the mesh 45 degres wich is useful if you have to erode a plane diagonally , like a mountain for example,avoiding lotss of jaggy edges , going on back and forth subdividing and unsubdividing make it easier to cut without having tons of vertices.