It DOESN'T MATTER...
- if you link back to the source and list the photographer's name
- if the picture is not full-sized (only thumbnail size is okay)
- if you did it innocently
- if your site is non-commercial and you made no money from the use of the photo
- if you didn't claim the photo was yours
- if you've added commentary in addition to having the pic in the post
- if the picture is embedded and not saved on your server
- if you have a disclaimer on your site.
- if you immediately take down a pic if someone sends you a DMCA notice (you do have to take it down, but it doesn't absolve you.)