Try this:.miostile {
background: url('../images/patterns/myimage.png') !important;
}
It doesn't have to be anymore complicated than that. Since you've created a new suffix, the code will only be applied to modules that use it.
Not sure why you added the .rt-omega - that wasn't in any of your code before - but you added it incorrectly. When you're using the web inspector looking at the classes, if an element has more than one class name usually only one of them is needed in most cases. If you need to use them both to specifically identify that element do not put a space between the names. So this code would also work:.rt-grid-4.rt-omega .rt-block.miostile {
background: url('../images/patterns/myimage.png') !important;
}
The rt-omega class is applied to the grid element, and module suffixes are added to rt-block - so there are only 2 elements. Your original code is looking for your suffix inside an rt-block that is inside an rt-omega that is inside a grid-4. In reality - there are only 2 elements that share 4 class names - not 4 separate ones. Make sense?