Click the down arrow of this button . Select New Image -> Blank. Create an image of size 16x16. We will make it smaller later by selecting only the part we need. We create it with a black background so that we can easily see it. Click OK to create the image.
Click on this button to open the Add Text Dialog.
Select "Webdings" for the font name. Click this button to open the Symbol Dialog. Select the triangular character as shown in the image below. The code for this character is the hex value F034.
Click OK to insert this character.
Set the size to 14 and the color to #FFFFFF.
Click OK to create the image. It should look like the one below.
Click on this button to enlarge this image a bit. Make a selection around the triangle as shown below.
Click this button to crop the image. It will look like the one below.
Turn off the layer with the black background by deselect the option at "Layer 0". The image should now with transparent background.
Click the button "Insert to Site" and select GIF for image type. Enter the name "nb-white-arrow.gif". Reduce the number of colors to 16.