This is a nice resource on using images on your site. There are suggestions for file formats and sizes, plugins to use, branding considerations, copyright and where to find royalty-free or public domain images that are ready to use.
The section on copyright law is a must-read.
Each photo could be different, each photographer is different, and the consequences of getting it wrong can be bad.
It basically seems works like this in most places around the world: when a photo is taken the person who took the photo owns the copyright to that photo. There are some exceptions (like journalistic photographers who are employed) but mainly it’s the gal or guy who took the shot that owns the rights.
That photographer can then grant licenses where other people can use that photo in certain circumstances and under certain conditions.
Adding images to your posts is a great idea. It makes a post more readable by breaking up big chunks of text, it adds visual appeal, and illustrates what you’re writing about. Setting a “Featured Post” is also important — a lot of themes for WordPress use these on the home page and category archive pages.