Most of the replies in this topic give the impression that you need Photoshop or other expensive professional quality software to repaint FS textures. I still say that's a load of nonsense & I'm sure that this rather snobbish attitude puts a lot of people off even trying it.
There are a lot of excellent alternatives available nowadays, many of which are completely free. Two freeware examples (GIMP & Paint.NET) have been given here but there are many others. I've never been able to figure it out myself but I've seen masterpieces created in MS Paint. It's a matter of finding whatever suits you. Most graphics editing software supports Layers which is very useful. They're not essential for repainting but most Photoshop plug-ins are fully compatible with Paint Shop Pro & other graphics editors, even IrfanView. (I'm not suggesting that IrfanView is suitable for repainting.)
A lot of the paint kits available now are in the Photoshop native PSD format so it would be useful to have an editor that can open this format. However, this is not strictly necessary for basic repainting. In the end it all comes down to what you're familiar with & happy using. There's no point at all in having expensive software if you have no idea how to use it.
Repainting aircraft was always a good way of getting started in this fascinating hobby. The best part about it was always that it costs absolutely nothing except your time & skill, so what's stopping you having a go?

PS. Just one example of an excellent freeware graphics editor with all the advanced features you could wish for. Ideal for repainting.
http://www.magix.com/us/free-downloads/free-software/xtreme-photo-designer/