Background image may appear only on wide screens

Hi. The way that background images are displayed will vary depending on what theme you’re using and how that theme is configured.

Could you be more specific about your setup? See also http://codex.wordpress.org/Forum_Welcome#Include_as_much_information_as_possible

If you want to test out what’s happening, you could also try uploading a much smaller image to see if it appears instead of the white background.