There are endless supplies of graphic editing software out there, ranging from the ones that you mentioned to less well known ones such as Serif Photoplus 6, PixelToolbox, etc.
In the end, for basic graphic designing (such as making signatures), gimp and photoshop are the best options in my opinion. You could try looking around some on google, if you haven't already, and see if you find another program that fits what you're looking for.
An alternative short term option would be to try out a PS trial (30 days) from their site:
http://www.adobe.com/downloads/