Split proxy for injection of ads ?

Actually I am using RoS-proxy to forward all traffic to a PC, injecting ads to finance my open hotspots. Although my SW on the PC can handle (simply ignore) all the traffic, into which the injection of ads makes no sense, like into .png, .mpg etc., it would lessen the burden onto that PC, and it would speed up browsing on the clients connected to the MTs, in case there is a possibility to filter the proxied traffic on the MT. Something like
“proxy only .htm, .html, .js”
OR
“Do NOT proxy *,flv, *.gif, *.png …”
Any such functionality available ?