Make transparent images, and use CSS to choose the background color.

statgit

@@ -7,7 +7,7 @@ import os
 import re
 import sys
 
-GNUPLOT_COMMON = 'set terminal png\nset size 0.5,0.5\n'
+GNUPLOT_COMMON = 'set terminal png transparent\nset size 0.5,0.5\n'
 
 def getoutput(cmd):
 	print '>> %s' % cmd

statgit.css

@@ -75,3 +75,8 @@ td {
 	border-bottom: 1px solid #cc9;
 }
 
+img {
+	border: 1px solid black;
+	padding: 0.5em;
+	background-color: white;
+}