$slice$.tga is a temporary file created during the slicing process and will be left on your disk until the next time you use Slice60, at which time it will be overwritten. It is actually a 24-bit TGA image of your original texture but with all of the swapped borders in place, and is therefore divisible by 64 pixels on a side. One possible use for this file is to load it into your paint program to alter it's colors or reduce it's color depth yourself, producing your own optimised palette, you could then save it as a .RAW / .ACT pair and then use GreenGiant's Axe for slicing into 64x64 chunks. Perhaps this is not the most preferable way to work, but the fact that you can makes us all happy ;-)

$slice$.tga is safe to delete.