How do you reverse sprites?

I'm trying to make a sig in Bannedstory, but I need to reverse Sartorius27 for it to work. How do you do that?

Note: All it is is a sig of Sartorius and a Windraider shooting each other, with a title I haven't yet decided between.

What program are you using?
If its part of the Adobe CS3 programs, it should be somewhere near Modify>Transform>Flip Horizontal.