Is there a way to modify the photo gallery templates to include a cusomt logo ?
Don't use web gallery myself but am afraid that the options for this in Bridge are very limited and they have not improved for several versions (CS6 also does not include a watermark for Web Gallery while the option is there in output to PDF)
So you will have to create your own watermark or logo. When you create an action to do so in Photoshop and create copies of this files you can use those copies for web gallery and save the originals. It is a bit work to create such an action (also Google for this for additional info) but once your action is working you only need to select the wanted files in Bridge, use menu tools /PS / batch / and choose your created watermark action and it all goes automatically