I don't know if this has been published before but I searched and didn't find anything. 

Thread.gif is the picture that is inserted before the member's avatar on threaded discussions.

On our network we use a grid paper background and our discussions are threaded.

The widget was invisible, making our most lengthy discussions a challenge to follow.

It took me a while to be able to ask the right question and finally get an answer for this problem, thanks to Chris on NC.

To change the thread.gif image for all your threaded indents add the following code to your CSS:

.i1,.i2,.i3,.i4,.i5,.i6,.i7,.i8,.i9{background:url(YOUR 20x30 PIXELS IMAGE URL HERE) no-repeat 2px 10px !important;}

I modified it to get a different thread image for each level of indentation, to help follow discussions that span many pages:

.i1{ background:url(YOUR 20x30 PIXELS IMAGE 1 URL HERE) no-repeat 2px 10px !important;
}

.i2{ background:url(YOUR 20x30 PIXELS IMAGE 2 URL HERE) no-repeat 2px 10px !important; }

.i3{ background:url(YOUR 20x30 PIXELS IMAGE 3 URL HERE) no-repeat 2px 10px !important; }

.i4{ background:url(YOUR 20x30 PIXELS IMAGE 4 URL HERE) no-repeat 2px 10px !important; }

.i5{ background:url(YOUR 20x30 PIXELS IMAGE 5 URL HERE) no-repeat 2px 10px !important; }

.i6{ background:url(YOUR 20x30 PIXELS IMAGE 6 URL HERE) no-repeat 2px 10px !important; }

.i7{ background:url(YOUR 20x30 PIXELS IMAGE 7 URL HERE) no-repeat 2px 10px !important; }

.i8{ background:url(YOUR 20x30 PIXELS IMAGE 8 URL HERE) no-repeat 2px 10px !important; }

.i9{ background:url(YOUR 20x30 PIXELS IMAGE 9 URL HERE) no-repeat 2px 10px !important; }

The images should of course be 20 pixels wide x 30 pixels high and can be either GIF or PNG, as long as the background of the picture is clear.