Irfanview ruuuules. I use it at work because it digs out more technical information from raster images than any other application, including Photoshop.

If I wanted to put water marks on photos, I'd use Photoshop. But You can create a transparent gif of your "mark" and then paste it on top of your photos and resave. If you just want your logo on there, just slap on it.

If you'd like help creating the first bit, lemme know. I can probably whip one up for you. =)