FreeImage does not detect and use binary transparency in RGB images like PNG or ICO.
actually the problem is only with PNG images - as transparency from 24 bit ICO images can be imported using ICO_MAKEALPHA loading flag.
FreeImage also does not detect transparency in Grayscale PNG, i.e. PNG Color Type 0, of various Bit Depth (a two-byte value in PNG file). What Lucian has mentioned is PNG Color Type 2 (a six-byte value in PNG file). Currently FreeImage covers only PNG Color Type 3. (For easy reference I will post a remark in the link given by Lucian).