Sarab,
That replacement code has no closing <div> tag... It is not valid HTML, I can't imagine it ever worked right as it is written- if it did it was probably just some browsers rendering it improperly.
However if you modified the bbcode like this-
Code:
<div style="width:100%; background-image:url({option});">{param}</div>
That would be valid/working code... You would use it like:
[divback="/path/to/image.jpg"]div contents here...[/divback]
You would need to set "Use Option" to YES in the BBCode settings.