jQuery
is the easier way.
Me, Google, Rendle, Mozilla, Microsoft, and thousands of others that aren't as important as these five all use and recommend it.
Anyhow... another thought - either for non-JS users only, or perhaps in general - have a pre-generated background.
Have a cron job that runs every X minutes to refresh it from the PHP script.
Perhaps you could even queue up several pre-generated images if you wanted to make it change for every page load.
This then allows you to sidestep the delayed loading altogether.
EDITED: 18 Feb 2009 00:18 by BOUGHTONP