You could do it using Paintshop Pro, Photoshop, Office Picture Editor... Probably in Windows Paint if you want to, but as others have said, there's no point unless you are planning on repainting them at that higher resolution.
I know one person who insists on using 4096x4096 textures, but then he thinks that 5-6FPS is perfectly acceptable to fly at so I really wouldn't recommend it!