It's 50/50 really. You're helping a search engine robot understand the content of your page, and at the same time you're maybe pushing the envelope a little much.
Why not change the background image to just an image on page with some title text? Alternatively, make sure the <h1> text is visible.
The text being invisible just raises eyebrows, even if it's totally related to the content on the page and the content is excellent.