Pretty much what the topic asks. How do you know whether using these pictures is legal or not? (Photographer having copyright/etc)
To be sure you're following proper copyright laws, just get the photos from a site that sells stock photos such as: Istockphoto.com, shutterstock.com, etc. Do a Google search for Stock Photography to get a list of providers.
Try fotolia.com, shutterstock.com some images in flickr is free to use as well. another source sxc.hu it is free
I use istockphoto. You can also get images from Flickr or DeviantArt, some are free and some are copyrighted, they let you know either way as some photos that are copyrighted can't be downloaded or copied.
try to visit icreatelogos site. I'm sure they will help you with your problems. They know a lot about designs and everything and i assure you they can be at your service.