test/css2.1/contribute: require PNG

Author: fantasai

test/css2.1/contribute.txt

@@ -11,9 +11,10 @@ The CSS2.1 Test Suite still needs a lot of work, and the CSS Working Group can't
 
 
 
+
 ===== Template for New Tests =====
 
-The baseline format for CSS2.1 tests is XHTML 1.1. A set of scripts generate the various formats from this source version.
+The baseline format for CSS2.1 tests is XHTML 1.1. Images must be in PNG format. A set of scripts generate the various formats from this source version.
 
 Copy and paste the following text into a new file, or save this file, and replace the capitalized parts. ''<link>'' elements are used to [[http://www.w3.org/Style/CSS/Test/guidelines.html#links|link to the relevant section of the spec]].