The CSS classes you have used are the correct ones, which are needed to set a new image for the RadSplitBar icon. I assume that you have missed to add the keyword !important
after your declarations - it is required to give a higher priority to your custiom setting instead of the default one.
I tested with the following code:
| background-image: url(alertIcon.gif)!important;
and the image was correctly set as expected.
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