You need to either use a larger image and not stretch it, or change a piece of code in the uifile. Some images stretch just fine, most actually, but some have the problem you are having. If you don't have a larger image to start with, open the uifile and change line 14 from this:
background: rcbmp(100,6,#0,0,0,1,0);//rcbmp(100,6,#FF00FF,0,0,1,0);
to this:
background: rcbmp(100,6,#FF00FF,0,0,1,0);//rcbmp(100,6,#FF00FF,0,0,1,0);
If you are creating the logon with the Logon Studio editor, you need to make that change as the very last thing you do, or at least don't open it with the Logon Studio editor again or it will change back and you will have to it again.
So create your logon, get everything the way you want, and then change the line in the uifile. If you don't know where it is, it's C:/Program Files/Wincustomize/Logon Studio/Your Logon.
Post back here if you have any problems or you can email me at boss0192002@yahoo.com