I get the "my images will just not work" part. As for the rest, I'm not sure. But what I'd like to know is how you makeHandleImageUrl="../T1818.jpgwork. Is there something more required to make an 18x18 image replace the default slider handle? Maybe we're asking the same question?
There are a couple steps to embedding the image with your extender.
First, add the image to your assembly as an Embedded Resource, just like the behavior js file associate with the extender.