As for the Georgia decals, that's a bit tougher, but not too bad, especially at your smaller scale. It could be as easy as finding good images and printing them on decal sheets. They won't be waterproof nor UV resistant, but that may not matter for an inside model. If it does, you can take the design to someone that is doing custom decals or waterslides and they can print it for you. After I was satisfied with my Hazzard County Sheriff decals I sent the file to the same place that I bought my General decals from.
As for designing them, I did my Sheriff decals in GIMP (it's an open source equivalent of Photoshop). This would be perfect for simple graphics and for cleaning up and arranging images that you find on the web, which will probably be high enough resolution to give good crisp results on a 1/25 scale. If you need to create some from scratch, a vector-based application would probably be easier, but I'm not familiar enough with those to give recommendations. Hoss's designs are very well done, so he would probably be able to give advice if you end up going that route.