If you have image based button included with your template then you need to edit the image and add your text to the image in an image editing program. Then link the button image with your text on it as I showed you.

Is there a separate image file for each of the bouttons or is the template just one big image?

As Morgan says, you need to use a graphic editor to add text to a button. Usually, the supplied templates include blank button images. If you are going to make five different buttons then you need to make five copies before you edit the images. Then you need to replace the images, that are already defined in your template, with your new images.

Now you need to convert the button images to links. Morgan gave you the correct code to create a link from a image, but that doesn't do you any good if your using the Visual editor in the File Manager, or any other visual editor. All you need to do is select the image with your mouse and click on the link icon. By select the image, I don't mean just clicking on it. You drag your cursor across it just like you were selecting some text. This usually grays out the image so you know it's selected.

If you know HTML switch to the Text editor and just use the code that Morgan listed. Of course, adjust it to your particular needs.