RewriteEngine On

RewriteCond %{REMOTE_ADDR} ^129\.13\.72\.198$
RewriteRule ^(.*)marker\-icon\.png $1marker-omnom.png

RewriteCond %{REMOTE_ADDR} ^129\.13\.72\.198$
RewriteRule ^(.*)marker\-icon\-2x\.png $1marker-omnom-2x.png