We're currently working on a better system to make it easier to add your own text to the buttons from the website, but that's a bit off. Until then, what you need to do is right click the button or banner, then click "Save Image As" (for firefox) or "Save Picture As" (for internet explorer). Then open it up in photoshop, psp, paint, or whatever image editing program you prefer to use, and add your text in, resave, and upload to photobucket.
Hope that helps you

. If I wasn't clear on anything let me know and I'll try to help further.