Thanks for the appreciation!
Just to make it clear, I figured this out myself and did not find this somewhere.
I am not too sure there is anything that will enable you to integrate it directly into the browser but I would stray away if I were you because even if you do manage to integrate it into the browser, you most probably won't be able to set it to be faintly visible and if you simply put it as the background, I am not too sure you'll enjoy browsing the internet because I am sure the picture will conflict with the colours on the page. Even if you somehow set it to be faintly visibly (like a watermark), the process will become very complicated if you want to decrease or increase the visibility of the image while browsing some site. If you use this method, however, you can change the image quickly and also set it's transparency to different values with just two clicks. There is also the added advantage of having a dynamic background.
Hope you understand my logic!
If, however, you still want to try to integrate it with the browser, I suppose you can experiment with the custom style sheets option in Opera.