for the life of me I cannot get this to work. I have a custom formua field call SIGNAL. Based on the YES or NO value in another field called Past_Due_Check_c  I want the SIGNAL field to display a RED or GREEN image. I continue to get syntax errors in the code as show below. Driving me crazy. can anyone help?

IMAGE(
CASE(Past_Due_Check__c,
"NO", "servlet/servlet.FileDownload?file=01570000000wgFz",
"YES", "servlet/servlet.FileDownload?file=01570000000wgG4"))