Rp Tutorials

I've seen a lot of people use header background pictures with a simple picture resized and placed on a background of the color of whatever they plan on making the overall background to (600 x100). While this sounds super easy I don't have Photoshop or any other program that could do this. My question is could anyone recommend a basic easy to use alternative that could be found online where I could make something like that? I'm not sure if this has already been addressed and I'm overlooking it or not. Any advice would be appreciated.