#posts background

Now this is kind of an iffy tutorial to make because each theme has it's own post code. 

Now if you don't know yours, or you don't have any of the codes i have listed below, then i would suggest searching {block:posts} and finding the div class before the {block:text}, IT HAS TO BE A DIV CLASS, not a div ID

Some common post divs are

.post

.posts

.box

Now after you have found the css to your post div, find the background code, it could be background: or background-color: and change it to this

background: url('IMAGEURL');