[Air-L] Images on websites

I am currently collecting data from the blogs of adolescent girls.  Many of the girls have used images in their journals that they have taken from some other source, sometimes sources that have borrowed from someone else so the origin of the image is unknown.  Can I use those images in my findings, if I can't cite the original source? Is there anyone who is using re-blogged images from Tumblr, because I see a similar issue there.   Thanks for your help....