Try to use your own photos. That will make your content more engaging and will take care of the copyright issue.
In case you can't use your photo, or it doesn't make sense, try to look for pictures with a Creative Commons license (for example, from Wikipedia), which are free to use as long as you credit the source.
Also, if you refer the readers to a news article from another online source, using a picture from the same article is usually considered "acceptable use" so it's okay too ( see my "One news item a day" posts ).