To change the width of the border (the line that’s drawn around all the squares), add this to the end of the URL:

&border_width=d

where d is the desired width in pixels.

To change the color of the border, add this:

&border_color=(d,d,d)

See the page on changing square colors for an explanation of “(d,d,d)”.

Here’s a chessboard drawn with a three-pixel-wide black border:

http://www.eddins.net/steve/chess/ChessImager/ChessImager.php?
fen=rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR/
&border_width=3&border_color=(0,0,0)