The Commercial Packages, some of these are very expensive!
Update: Feb 2013, Adobe have released some programs as possible "abandon-ware". Read about that here.
You don't have to have these and don't worry if you don't, there are other alternate programs you can use. But if you are lucky enough to have some listed below here is some info about them in regard to editing Half Life Graphics.
Adobe Photoshop is great to use on Half Life 2 games, not only because it is the most commonly used but also because of a special Plug-in that was made for it... a VTF Plug-In for Photoshop 6.0 and up at Nem's Tools.
The Plug-in makes it very easier to change and save the native format used in HL2 games the VTF Format, once you have a few things setup on your PC you will be able to click on a thumbnail, Photoshop will open and after you finish editing the file, save as a VTF.
If you have read through the page in the VTF Plug-in link above you may see that Nem and another person has also got it working in Paint Shop Pro 8 and 9.01. If you have another type of graphics program that uses plug-ins it may be worth looking into...
Taken from the comments section of the link...
Post 6. ekramer
will it work for photoshop 5?
Post 7. Nem
No. Adobe charges money for their SDK (currently v7). It took me hours to find the old v6 SDK on the web, which was their last free release. It only supports Photoshop 6 and up.
You could copy and paste from Adobe to VTFEdit (easiest solution), the alpha channel is not supported with this method however.
Edit: A few people have mentioned to me that it does work in Paint Shop Pro. To install it, create a folder in the PlugIns folder called VTFLib and place both the VTF.8bi and the VTFLib.dll files in it. If it works please let me know which version of Paint Shop you have, I know it at least works in Paint Shop Pro 8.
Post 20. BadPrankster
Adobe Photoshop Thumbnail problem
Other commercial Graphics Programs
So other top commercial Graphics programs like Paint Shop Pro by Corel can be used but one thing to remember is the Plug-in Nem wrote was only for Photoshop 6 and up... but, it does work with some versions of Paint Shop Pro so investigate that further.