Hi guys, I don't know if someone already mentioned that, but personally I use
Paint.NET (the official successor of MS Paint) for nearly 99% of my artwork because it's very intuitive, simple and user-friendly. Best of all, it's absolutely FREE, multi-language and you can also download many additional effects and tools from the official forum (most of them are well explained in its own topic) by simply saving them into the program's "Effects" folder.
I know that Photoshop has more tools but it's too expensive option for those who need to draw only few custom buttons and backgrounds for Mach3.
Here are the links to the main website and forum where you can find loads of helpful topics, tutorials and effects:
http://www.getpaint.net/
http://paintdotnet.forumer.com/index.php