Cover a Watermark with ANOTHER Watermark on ALL images

I own an escort directory which includes many ads that have a graphic/watermark on the bottom right corner of each picture within the ad. I am not sure if this is possible, but I need some way to place my own graphic/watermark OVER the watermarks which are already on these photos. I believe this can be done by modifying a bit of code, but I am not sure. I need for ALL current photos to have there watermarks covered AS WELL as any FUTURE ads. I obviously need this to be an AUTOMATED process. Recently some one told me that this can be done by editing the stylesheet. If this is how it is done, then once you have finished the project I would like for you to explain to me how to find this stylesheet on my site and explain how I can make this change myself in the future. So that if I wish to change the graphic in the future I can. I will supply you with the graphic which I intend to use to cover the watermark. MUST BE 18+ to ACCEPT THIS PROJECT. Thank you!!