Thats a pretty good question. You can try both and see works for you. Also make sure to test it in all browsers that you may have because I've done one thing in Firefox and it not show correctly in Internet Explorer, esp. with images.
I used to add them in the html but have started trying to add them in the css. 1 part I seem to be missing out on is that I can no longer add alt attributes to images that are in the css.