You don't need the download - open up a command shell and type "imgcvt -h" - this gives you the help text for a utility that will do what you want.
For your sequence, open up the command shell, cd into the directory containing your image sequence and type:
imgcvt -n 1 100 1 name.@.tga name.tga.@
(the orange underline is NOT a link - the "@" stands for unpadded numbers - BTW, does anyone know how to turn that auto-feature off?)