Does anyone know how to insert a background from other sites ??

for instant: if I want to have a background on this page , what should I do ???

You can usually right click and 'save background as' on most sites. That should work, although sometimes it can be tricky because of the complex table structure some sites use. So careful you don't save a spacer.

not really clear to me !!
Right now , I have a background source code , then how I insert it on this page ?? Is it possible to do that ??

If I understand what you're asking:

1) You can't put a background on a web page unless you're coding that page. You can't change the page in your browser, you have to change it on the server (or upload it to the server).

2) Be very very aware that if you get an image from a website, it's copyrighted. You don't have permission to use it unless the website specifically says otherwise.

I want to insert a background on my post only

Oh, I thought you was talking about adding one to a website. You can't customise post backgrounds on the forum unfortunatley.

Thanks