There are some great freeware programs out there, and I certainly do not think any sane person would spend $1,000.00 for a paint program (thats ludicrous, unless you are a millionaire and do not care).
But... Photoshop is highly recommended. I love it.. (I LOVEEEEEEEEEEEEEEEEEEE it). But I have an ancient issue, Photoshop 6.0.
Some people sell it on Ebay, versions 6 or 7 are good.
I purchased Photoshop Elements last week for my Mac (mac version of PS) and it is brilliant. I am really impressed with it. I would check to see if you can use the nVidia tools with it (Elements) and if so, you can have the lastest Photoshop for $99.00. If you can afford $99.00, its worth it. It does layers and effects, etc, and is pretty nice compared with PS 6.0. (I may have to upgrade my PS to Elements if it can do the nVidia tools for when I am in Windows mode).
The cool thing with the nVidia tools is that you can double click right on a DDS file in FSX and it opens right up in PS. You can save as PSD and then to DDS in PS which makes PS ultra useful in doing heavy painting in FSX. For converting to FS9 formats, I just use DXTBmp by Martin Wright. Great program. It would be cool if he could figure out how to do a addon script/tool for PS that would enable DXT format saving, like nVidia did with DDS textures. That would be the ultimate painting setup.
The main thing of all of this, with PSP, PS, Gimp, is the ability to do layers. If you do not know about Layers, they are basically 'stacks'. You see from the top down, so some top layers will hide bottom layers. You can turn off layers, give layers effects, group them in folders of layers, etc, but are all stacks. From a stack, you can burn a JPG, BMP, DDS, etc, from them, but keeping the stacks 'stacked' and not flattened. Thus you can insert a layer of different stripes (stripes only) over a white background, change the background to beige, green, etc. Thus, stacks are totally amazing for repainting. Some layers can have just rivets, some have panel lines with effects for shading, etc, etc....
Great fun painting in layers..
Bill
LHC