Creating a whole new custom skin for such a minor thing is highly unnecessary.
Judging by your screenshot, you are using the Vista skin and RadControls Q3 2008 or earlier (correct me if I am wrong). In this case, the CSS rule, which should remove the pager background image is:
If the above does not work, then one of the following is true:
You are not using the Vista skin - in this case please replace "Vista" with the actual name of the skin
You are using RadGrid Q1 2009 with a migrated Vista skin for Q3 2008 - in this case the following should work:
You are using a modified skin, which contains some styles that I am unaware of and they add the background image - in this case I will need a runnable example with the skin in order to help you further.
Let us know how it goes.
the Telerik team