Let me see if I understand you correctly... You want to have an outline around some text that you are positioning over a background image via CSS? If I do understand you correctly then, I don't believe that is possible with plain CSS. You could always add the text to the background image and add the outline in your imaging program.
However, if you wish to get more complicated, I think I have an idea. You could have two layers with the same text. Position one above the other via CSS positioning and z-index. Then make the text in the back a larger size than the text in the front. That should give you the effect of an outline.
Does that help at all?