Hey guys i found this out yesterday! you can use a skin when playing offline. Okay if you want to use the default texture then go to %appdata%. next you will go to .minecraft and go to bin and open minecraft with winrar. then go in mobs look for char.png and delete that then look for the skin you want to use. rename it char and move it into the mobs folder.

if using texturepack go into the texturepack and look for mobs and do the smae thing :D

any questions? put in comments.