You could use a pixel shader to identify contour pixels of your sprite. I do that to make them black (in order to
have a better visual segmentation of sprite/background, even if in the end i do that only off-line). Once you have
such functionalty (which, however, will work only with opengl es 2.0 or higher), you can draw a red border, for example, around
your sprite easily or change the original border or add a thicker border.