Another one I recommend is Adobe Photoshop Elements. While the full-blown Photoshop is the gold standard of image editing programs, it is very expensive and its cheaper cousin, Elements, is fully adequate for decal design. At the time I wrote the article, Paint Shop Pro X4 was out, but full of bugs, and I couldn't recommend it, so I
recommended trying to find X3. However, they have since come out with X5, which got rid of the bugs in X4, so I can now recommend the latest version. Both Elements and PSP X 5 are in the sub-100 buck range, typically about 60 to 100, depending on store and whether they are on sale.
Another option is a cheap CAD program, like TurboCAD, but they do not handle imported pictures as easily. The advantage to CAD programs is that they are easier to do a precise resizing to fit the scale of your modeler. Image editors do allow good resizing, but it is just a bit easier in CAD.