An Image in the showthread
 

I would like to add an image in the background for every users that replies or creates a thread.

How do I accomplish this?

An example on this one, behind my text will have an image (which you cannot right click and find the path to that image of course). How do I do that?

you cannot hide the path to an image

to add a background image to a post use CSS

if you want to use a different image for each post I suggest using inline CSS ie. if you want one image for all posts ..
create a new class and add it to your stylesheet via Style Manager / Main CSS

you can then prepend the new class with the alt1 selector of the postbit
ie. class="alt1 newclass"

Thank you very much.

I'll try this after work.