BuddyPress.org

You may use this plugin: http://buddydev.com/plugins/bp-custom-background-for-user-profile/

And change

<style type="text/css">
body.is-user-profile{
    background: url(<?php echo $image_url;?>);
	background-repeat: <?php echo $repeat_type; ?>;
}
</style>

to

<style type="text/css">
.my-account div#item-header, .my-activity div#item-header{
background:url(<?php echo $image_url;?>);
width: 100%;
background-size: 100%;
background-repeat: no-repeat;
}
</style>

in the bp-custom-background-for-user-profile.php file.
It may not work good with every theme!