Try to save the file as head.tga head.jpg or head.png in the folder with your name on. Often its slow with the loading so try to go in and out of a multiplayer server or something, and go back to sp.
I figured this out the hard way, i made a whole textured charater and then when my internet came back on, my item.dat file was redownloaded and i looked like uke... *sob* *sob*
I'm trying to texture uke's knees but when i put the images in the folder with the names j_l_knee and j_r_knee they won't appear ingame. And they are in TGA format.