I'm guessing that the graphics you are looking for are all of the ones on that page that have anything in their titles about the chat input box. It looks like there are ten of them.
- UI-CHATINPUTBORDER-LEFT.png
- UI-CHATINPUTBORDER-RIGHT.png
- UI-ChatInputBorder-Left2.png
- UI-ChatInputBorder-Mid.png
- UI-ChatInputBorder-Mid2.png
- UI-ChatInputBorder-Right2.png
- UI-ChatInputBorder.png
- UI-ChatInputBorderFocus-Left.png
- UI-ChatInputBorderFocus-Mid.png
- UI-ChatInputBorderFocus-Right.png
If it is not necessary to use a blank graphic for all of them, I'm sure someone will jump in here and correct me. 8^) You can see the graphics by clicking on the "Show all images" link at the top of the list. In the bottom row you will see what appears to be a blank space next to the single edit box graphic. That's not a blank space, it's the three "Focus" graphics.