I try to put this picture wallpaper as my profile background here but it says this:

The image must not be larger than 512000 bytes.
my bg: http://i.imgur.com/lsSIODF.jpg please someone tell me how to use this?

You could reduce the picture size with paint or another program and upload again

You can also print it as a PDF onto your desktop (A virtual print) that will also reduce the Size of the picture.

Try this guide:
http://forums.d2jsp.org/topic.php?t=68613110&f=246