YES, You Can Be Sued.

Roni Loren, writer, describes her adventures in having to settle with a photographer whose copyright she infringed on her blog: You Can Get Sued For Using Pics On You Blog.

...most of us thought if you added commentary, gave attribution, etc., then it was okay. And sites like Pinterest and Tumblr being so popular reinforced that feeling that it must be okay because otherwise--how do those sites even exist, right? Picture sharing is the whole point of those sites.

The blog post and the comments below are a recommended read.