This can be done using a module class suffix like horiz-bottom
then add this at the bottom of your template.css file:.horiz-bottom .rt-block {background: url(../images/style1/horz-div.png) 50% 100% repeat-x;}
that will do if it is just one module on top of one module
If you are talking about multiple columns of modules you can do something like this.horiz-bottom .rt-block {background: url(../images/style1/horz-div.png) 50% 100% repeat-x;margin-left: -10px;margin-right: -10px;}
but beware that using different title types for module class suffixes or content amounts will mess with the height and thus the alignment.