Normally, you just select mapped picture in the bump tab, choose you image, and voil Change bump strengh if need be, but that's about it. Use a filer if you want, you can use the "bump quantize" from "other filters", it prevents artifacts. The only other solution I see is to use the image offset parameters in this tab. The scale for this tab is in pixels, so values of 100 or greater are generally needed. All I can see.