What level of image editor and/or paint program do you need? Keep in mind there are 2 separate but related usage types here, filter-oriented image editing (XnView works more this way), and more painting-type stuff, i.e. manipulation with brushes, etc. Photoshop combines these two very effectively and there are other apps that do similarly, but if you only mostly use one or the other, there may be good free or cheap options.
For example, free options include:
Google Picasa is good at image organizing and filter-based workflow. Mostly good for real-world photo collections but could be used on TG work.
Paint.net is a popular option. The painting options are somewhat limited, as are the filters.
Photofiltre does both adequately.
The GIMP is a powerful app that combines both aspects, but has a somewhat clunky UI (depending on who you ask). It's probably the most direct Photoshop replacement you can get for free.
For pay, maybe Paint Shop Pro?
- Oshyan