Preferably you need a prog that will allow the inclusion of alpha channel to your textures, some progs wont do this.
I dont use TGA at all and use DDS (with alpha where necessary) for all textures that I make and convert any that I dont. I dont see any advantage in using TGA. Just my opinion.
Personally using TGA would make my level textures eat up much greater amounts of memory and make overall level sizes for compiled levels too unmanageable. I use a lot content and variable textures to view - and memory display requirements are massive.
Converting from some other file formats wont always retain alpha channels needed for transparency where necessary.
None of the paint progs require a huge level of brain power, a little time experimenting will soon show you what can be achieved. If a prog cant do what you want get one that can.
Photoshop is obviously the best choice for working with textures, though its useful to have as many as possible - there are some things that one cant do that another can. You cant have too many progs.
If you want good progs - then its Photoshop, Corel Photopaint, Macromedia Fireworks, Paint Shop Pro amongst others I guess. There are loads of freeware/shareware paint progs and convertors around too - search on Google or get them on Free Computer magazine cover disks. (well you gotta pay for the mag, but you can pick up some good fully working progs if you search around)
Try them all and decide which you like. We all have preferences.