I am working on styling the searchform for my new theme, but I want the image for the advanced options to be a css background so I can use rollovers. Right now I would have to modify the printSearchForm() function in template-functions.php to accomplish this and I really don't want to (looking into the future it's not a good idea). A simple preg_replace could fix the output but there is no way to simply return the outputted html, it simply prints it.

Could an option to return output rather then echo it be added to the function?

Is there a better way to approach this that I'm missing?

There is a lot of HTML generated by that function. I think you would be better off creating a new function that generates the HTML you desire. Or just style the tags that are created by the function.