The ALT tag is added by WeBWorK. I wasn't suggesting that you make the change (I was recommending it to the other developers who read this list). The file that does it is pg/lib/WeBWorK/PG/ImageGenerator.pm, and I'll add the escaping when I get the chance.
If you plan to change $GT you should instead change $GTS in pg/macros/PGbasicmacros.pl by changing the subroutine definition of GTS around line 1244.
The browsers I used to test the PG problem file you site showed the IMG tag fine even with the unescaped >, so perhaps there is something within the WeBWorK/Moodle connection that does it. I suspect a bad regular expression somewhere that doesn't properly detect the end of the IMG tag while altering the output of the WeBWorK problem.