you must be importing a image (tga) with cube size, ex; 32*32px, 64*64px, 128*128px, 256*256px 512*512px and etc.
and I suggest save them with the fallowing name;
[YOUR_NAME]_sp.tga , [YOUR_NAME]_ori.tga , [YOUR_NAME]_mk.tga , [YOUR_NAME]_sp2.tga etc.
Edit:
and use gildor's or elfocrash's tool so that you don't need to have time to rename them.