While copyright can get very complicated, fundamentally it is usually pretty simple. If you do not own the photo, than you are not free to use it.
There are exceptions. Sometimes people explicitly give permission for their photos to be used. There are also fair use rights, which includes things like satire, criticism, news reporting, etc., however, fair use isn't an automatic protection. You could still have a legal case brought against you and have to justify your fair use claim.
For photographs of people, copyright isn't the only concern. People in photos, who often may not be the copyright holders, still have a right to privacy; and for celebrities, the right to publicity (the right to control how their image is used).
For a general blogger who isn't making money off of the photo, the worst that would probably happen is you will receive a DMCA take-down notice. Here is some good information on common copyright issues for bloggers and on the DMCA: https://www.eff.org/issues/bloggers/legal/liabilit...