Can you expand on your requirements : )
As I see it you want to use "http://www.mysite.com/image.jpg" as the background. On the background you want a white box and some text in the box.
If you use ImageMagick rather than GD you could add a white box image to the original image and then add some text to that image. Would probably take two passes at acheving this but you could use a temp image.
I know a good example of ImageMagick and would use it in preferance to GD; you have also caught me at a bad time as just removed my website to start again and have deleted all the things I tried.
Anthony
http://www.cit.gu.edu.au/~anthony/gr...ating/#overlay
P.S. PM me if you need some help