replace background or any image from a web

[Randy Medina]

Hi, im from Peru and i try make a userscript for (greasemonkey=Firefox ; Tampermonkey=Chrome).

The idea is: With "JavaScript" search the background image url (http://www.bgimage.jpg) and replace with another (http://www.myownbgimage.jpg), it is possible?

Plus, search and replace a especific image in that webpage for another image.

Thanks for read and sorry for my bad english.