This is how you create a super basic submit button:

<input type="button" value="Submit" />

All you need to do to replace this default button with an image is to change the type to image and then declare a source image like this:

<input type="image" value="Submit" src="submit-1.gif" />

You are also able to add a class declaration if you wish to further style it (e.g. floats, borders, etc.).

I have made up an example page with a bunch of example submit buttons. Also feel free to download the example page and do with it what you will.